This is a question I get frequently from friends, family, and fans. I decide to lay out some numbers for those who are looking to make the jump and become a full-time writer. To make it easier to read this post will assume you have no other income and your only income is from books sold or read on Amazon.
The first question is how much money you’ll need to live. I’m going to take the average US personal income of $31,000. If you need less or more feel free to adjust the numbers to see fit.
The second question is how much can you sell your books for. Most new authors will try to sell their book at either $0.99 or $2.99 for their books as they want it to be appealing to readers. 99 cents because they want to “sell as many as possible” and $2.99 because that’s the cheapest you can go and get 70% royalties. This is also the value that Amazon will try to get you to price your book. 
One thing to understand when you sell a book at $0.99 is the volume you would need to sell to earn *any money*. 100 books sold at $0.99 would earn you $29.70. To earn that with a $2.99 book you’d need to sell 14-15 copies. Now, which do you think is harder? Convincing 15 people to buy your book or 100? Especially once you start considering advertising costs the higher price book makes more sense. I personally don’t list books at $0.99 or even $2.99 unless I’m having a sale. My writing is worth it and I sell them for $5.99 or more. Which means I only need to sell to 7 people to earn as much as selling 100 copies at $0.99.
Another thing to consider is Kindle Unlimited. Even if you’re selling your book at $5.99 you may only earn $1 to $2 per read for your book depending on length. People may lose interest, something may come up, and your earnings fall even lower than if it was a sale. For some genres Kindle Unlimited makes sense. For others, not so much. Going forward I’m just going to assume we’re doing a wide distribution and selling books for the full price.
So how many books, priced at $5.99, do you need to sell to earn $31,000 a year?
Well, you earn $4.19 per sale. So you’d need to sell 7,393 copies. I’m just going to round to 7,400 to keep the math cleaner.
Just 615 books per month.
How hard can that be?
Well to do that with ONE book you’ll need to be ranked around 6,200 on the entire Kindle store. And you’d need to maintain it for the entire year.
So you write a THREE book series and they all sell equally. To earn your goal you’d need to be ranked around 20,000 for the entire year for all three books.
Five books? Rank 30k
Ten books? Rank 60k
For many people maintaining a rank above 100k for a year is a challenge. For others, writing is the hard part. Even books that have been in Amazon’s top 100 have fallen to above 100k after a period of time. So it isn’t easy at all. Think about the amount of effort to put out one book and imagine how much time and effort it would take to put out 10 or 20 books.
Ultimately, the “how hard it is” comes down to your approach. I’m a quick writer and a terrible marketer. So I focus on quick releases of many books. Others are very charismatic, great sellers and marketers. They can earn the same revenue from one book that I earn on 10. But, the trade-off is it takes them as long to write one book as it does me to write 10.
Finally, the numbers above are revenue. Each book has costs associated with it to get it to market. Additionally, you’ll need to spend money on advertising and other promotions to get your book in front of others.