While most of this comment is true, there are lots of youtubers that can make it full-time without getting millions of subscribers. The key is that you can't rely on adsense, you need to get as many different ways of getting income as possible (patreon, sponsors, merch, etc).

From my observation, it seems an active base of around 200,000 subscribers seems to be where you can do it full-time. I've even seen people with about 100,000 subscribers go full-time.

The trick is that you can't just be making videos, you have to take on a lot of the business parts too. If you just want to make videos and nothing else, then you would probably need hundreds of thousands of views per video to make a living.