You’re buying it from people who are selling bitcoin. Those people either got their bitcoin from mining it or buying it from other people who sold it.
There’s two types of limits for bitcoin, so I’ll address both.
- You can only buy as much bitcoin as is being sold. Some exchanges have more available bitcoin compared to others. Unless you plan on buying billions of dollars worth of bitcoin, you probably don’t need to worry about this limit.
- You will only ever be able to buy 21 million bitcoin, that’s the total amount that will ever be created. Currently there are only 18.5 million in existence, so that would be your theoretical limit (though there isn’t that many being sold currently).