If an author sold five million books is he or she considered successful

Why is John Green only worth $5 million? The books he wrote are extremely successful, and the first movie of one of his books made an immense amount of money in the first week alone. He gets a lot of buzz and has a highly successful YouTube channel.

Writing doesn't pay much.  To have $5 million, and some estimates put him over $10 million lately, is incredibly successful.  He is perhaps poorer than his level of fame might suggest, but that is easily explained by his own altruism.  Much of his fame is from non-profit endeavors.  The Vlogbrothers channel doesn't generate income directly for the Greens, and when it is used to raise money it goes toward charity and to support other projects.  VidCon as well probably doesn't make any money for the Greens but is instead reinvested in the conference and video promotion.  However, that indirectly increases the fame and influence of the Greens, allowing them to earn more from books, albums, etc.

If he wanted to make more money, he could.  The problem is that he chooses not to abuse his influence for personal gain.  He is comfortable with the immense success it already provides him and instead focuses on improving the community and decreasing world suck.

How many copies does the average book sell?

The average sales per title are utterly without meaning. There are vastly different numbers for mass market paperback novels, trade-paperback original novels or trade non-fiction, hardback original trade books, and for non-trade titles in any of a hundred sub-categories. I can only speak about the US, but here's how you find a number that MIGHT mean something to you.

There are about 50,000 trade titles from mainstream publishers per year. They'll sell between 1500 copies (a disaster, but it happens a lot) and a few million (which happens a handful of times per year).  An okay sales number will be 5,000 to 15,000 copies.

Mass market paperbacks must break 10,000 copies per title to break even, and they're very high risk, but also high reward if things go really well (which is really rare). That's why only a few of the largest houses do them.

THEN there are self-published books. The numbers for these are FAR worse. They come in three basic groups.

There are the pros who are taking charge of their own careers, and who are doing their homework on production, covers, layout and text design, marketing, distribution, and so on. They may be in trade fiction, trade non-fiction or non-trade, but they have their own publishing companies, and you'll be hard-pressed to tell the difference between their books and the books coming from any large NY-based publisher. This group has a few thousand titles per year, although it's growing. They sell about 1/3 as many copies as the same book would through a mainstream house, but the author makes much more per copy, since most of the sales are ebooks, through Amazon, and they're getting 70% of the sale.
There is the newbie dreamer, who believes their friends and relatives, and thinks that their book is great. They hire a so-called self-publishing company, and their books sell a few dozen copies, and stop. This group is huge.
This is the largest group of titles, but with the fewest practitioners. They're using computers to scrape copy-left content off of the web, assemble it into ebooks and harvest a dozen or so sales per title across thousands of titles.

I hope that this helps you understand how many copies a "bookstore-type" book might sell in the US.

If you're not in the US, you need to pro rate those numbers. No other country's book publishing industry is more than a third the size of the one in the US.

If an author sold five million books, is he or she considered 'successful'?

I'd say yes, but in reality, a writer might have to write dozens of different books, over the course of his lifetime, to hit that number.

How much does a book writer earn in India?

It depends.

When it comes to writers, they face a similar situation faced by other artistic chaps. There are writers who earn a ton of money, and then there are writers who have to take up either a secondary part time / full time job to meet their expenses. And there are writers who don't earn huge money, but can meet their daily expenses without the need to juggle another job.

New authors get anywhere around 5% - 7.5% of MRP as royalty for their books. That is, if that book costs ₹100, then the author gets ₹5 - ₹7.5 per book sold. And if that book sells a million copies, the author can earn upwards ₹50 lakhs for that book; if you sell a million copies, that is.

According to this source, among the hundreds of books that come out every year in India, those that cross 20,000 sales can be considered 'successful'. Indian books, in my experience, are priced somewhere between ₹80 - ₹200.

Being optimistic, and picking ₹200 as our MRP - let's do the math. If you sell 20,000 copies of a book that costs ₹200 at a royalty of 5%, you earn ₹2 lakh. But wait - you must first get your book published. Publishing houses rejects tens of thousands of books all the time. Once your book is published, it must sell. Which means that the probability that you'd get more than ₹2 lakh is very, very low. As mentioned before, only the successful books cross the 20,000 sales mark. There are a lot of average, slightly above-average, good - but not really successful and poor books in the market. And a writer would earn much less for those books.

If you've made your mark as a writer, it's quite easy to earn some big bucks. If not, well keep your day job at the bank, or at the software company - because I don't think you'd be able to pay your bills by writing books in India.

How much do authors actually make for each book sold?

Almost nobody gets books published regardless of whether they have been to college. I'm not aware of _any_ college courses in how to write bestsellers. No, you don't need to go to college to become an author. But you do need to write better than 99.9% of people who have. Probably add a few more 9s to that. You want to write? Then write. Either you'll have fun doing it as a hobby, or it will be good practice to set you on the path to being a pro. There's no downside :) There's also no jump start to getting a book published. You start off by writing short things which are not publishable quality. Everyone does. You get better with practice.

Is there a way to know how many copies of a book have been printed and/or sold?

The only 100% accurate way is to ask the publisher, and they probably won’t give out that information to members of the public.

In terms of knowing how many books were printed, only the printer and the publisher will know that. It’s not a piece of information that goes out into the public domain in most cases (unless the publisher is using it as marketing - this book is so great, we’re printing a million copies!)

In terms of knowing how many were sold, there are two measures here, sell-in and sell-through

Sell-in is how many copies were bought from the publisher by retailers
Sell-through is how many copies were bought from retailers by customers

Only the publisher knows the sell-in figure. The sell-through figure is reported by most retailers to Nielsen Bookscan, a private company that tracks book sales. If you have an account with Nielsen (which is subscription-based), you can query their database to get sell-through data across any book with an ISBN and which is sold through the retailers they cover.

Despite being called TCM - Total Consumer Market - this coverage is not actually Total. It doesn’t cover things like book clubs, catalogue sales, and various other sales that aren’t via book clubs or online retailers like Amazon. It also doesn’t cover ebooks.