There is no set limit like with the match or residencies. New schools can open up, existing schools can increase class size or run additional classes to match demand, and they have the incentives to do so.
The match can do that, too. 2019's had 6% more, because more programs joined in. There's no set limit; there's a limit to how many slots Medicare subsidizes, but programs are free to offer self-funded slots over that.