45 day visa exempt.
I'm not being pedantic, it's a very important distinguishment in the context of your question.
For visa exempt there's a limit of 2 by land per calendar year. There's no hard limit by air, but IOs can get picky if they don't like your entry history; it's always a gamble going in visa exempt if you've had more than a couple in a row.
For entering with a visa, obtained either at a consulate or online if applicable to your country, there is no limit. It can start to get difficult obtaining said visas after a while, though.