Do I need a visa for Thailand if my trip lasts 2 months and includes visits to Vietnam and Cambodia?

Your profile suggests you may be Canadian. If so you won’t need a visa to enter Thailand for stays of less than 45 days (before April ‘23).

You can indeed enter once or twice with probably no issues. Trying to do so more times could potentially mean an immigration officer pulling you aside and asking you why you’re trying to stay long term without a correct visa.

As

@Bra****

mentioned you can only do two land border entries visa exempt in a calendar year.

No! you! do! not!

It depends on what passport you have. If you're from a visa exempt country, you'll be stamped in for 45 days each time you enter.

If you're going to be crossing land borders, then you can only use visa exempt entry at a land border 2 times per calendar year.