After beating the game, you have the option to buy more power moons from the Crazy Cap store for 100 coins. I bought 15 of them at Mushroom Kingdom... but it doesn't seem to stop. And it doesn't seem like this count carries over to other shop locations.
Is there a limit to how many power moons I could buy?
No, there is no limit. You can do this at any store that sells these as well, giving you more power moons than you would ordinarily find in game. I suspect this exists to offset the harder-to-find power moons, giving you the ability to keep exploring post-game content without having to find every power moon.