I guess you could ask the question this way, "How many copies of Minecraft did Notch need to sell in order to be profitable?"

I kind of think the answer is hard to define though.

I doubt you find many indies at that level. Really, indies live off of a few 10K. And that was actually what Yacht Club Games was expecting for Shovel Knight:

But what isn’t obvious is that we knew that Shovel Knight would be a success. [...] What concrete numbers do we speak of? We’re talking the 15,684 people that supported us during the Kickstarter campaign. Other developers have estimated that preorders predict first-week sales by 200-400%. That means Shovel Knight was guaranteed to sell 2-4 times the 15,684 backers in the first week, or in other words, 30k-60k copies in the first week. That of course seemed crazy to us…as successful games in our portfolio barely broke 50k copies sold.

https://yachtclubgames.com/2014/08/sales-one-month/

So they saw 30K-60K as a guaranteed success for the game. So, with two million they sold actually 40 times the amount they considered a success. And notice that they said there portfolio barely broke 50K copies. That is true for most indies that hadn't a breakout success.

That depends on a variety of factors, like the platforms, the size of the game, the budget in general, the type of publishing deal the developer has and many others. Some could begin making a profit as early as 50,000 copies sold, while others might require several times that. There's no clear-cut answer.

Certainly a dumb question. Indie games have smaller teams and the team sizes vary a lot. Then there is the question of assets, some may already exist, some may need to be bought and some may need to be created by a hired person. Then of course the size of the game, the amount of models and design that has to go into it. So on my estimates an indie game will cost somewhere between 100 and 5m dollars.