Lets assume I someday make a game that's good enough to sell.
How many would I need to sell?

There's two main factors to consider.
Time and money.
How long would it take me to make that game?
How much money do I need to pay my bills?

Lets assume for now it's just me using free assets with the occasional asset bundle.
I need something like 1500 to 2000 euros net income per month to cover my living expenses.

So if the net profit margin, after taxes and all that, is say, 2 euro, I would need to sell 750-1000 copies every month.
That's 9.000-12.000 sales a year. Lets round that to 10.000.
Selling 10.000 of anything seems like a pretty tall order.
If the net profit margin were 4 euros, I'd need 5000 sales a year.
8 euros,  2500. 16 euros, 1250. And so on.

Do i feel like I can make a game that sells for enough to make a 16 euro net profit?
No. Lets be real here.
At best, I could maybe make something that sells for 8 to 15 euro. 
And that's a stretch. 4 to 6 euro is more likely.

I havent sold any games yet, but lets assume I keep 30% of the sales price.
30% for the sales platform the game is sold on, and the tax rate in my country is 50%ish, so half of that remaining 60% would probably go to taxes.
Those numbers might be wrong, but some quick ecosia-ing seems to point in that direction.
For now, lets assume i keep one third.

So if I sell it for 6 euro and keep 2 euro, I need to sell 10.000 copies a year to cover my living expenses.
A slightly higher sales price would help a lot. Keeping 3 or 5 euro would cut down the required amount of copies sold.

For now, lets start with the goal of selling one game at any price, and keep the day job.

Selling 10.000 copies a year, with a net profit of at least 2 euro is  a nice long term goal to works towards though.
That would be a neat milestone. Then I could call myself a developer. Now I'm a tinkerer? Hobbyist?