Can ASEAN citizens enter and exit Thailand as many times as they want in a year?

For countries bordering Thailand you’ll find special arrangements for citizens to cross. Outside of the scope of this group but basically they can come and go freely using those borders. Many citizens of say Malaysia or Laos work across the border and return every day. Special rules for those.

You can enter 2 times using visa exempt at a land border per calendar year. Most of the nearby countries do not qualify for visa Exempt so the question is irrelevant.

The citizens of some countries may qualify for APEC card and can enter that way, but that's not visa exempt.