What are the visa entry requirements for Thailand after multiple tourist visas and visa exemptions?

What Brandon said.

After this entry, you'll have been here over 6 months on tourist visas and out-then-back-in visa-exempt entries.

That's the point where you have a fair likelihood (not to say this definitely *will* happen, its all down to the immigration officer working with you) of being questioned about what you're doing here, and potentially either denied or told "last time".

All you can really do is have the three proofs of finances (ideally 20K Baht in cash, though officers have been known to accept bank statements), of onward flight ticket, and of accommodation.

That and a story prepared on what you've been doing here these past six months.

You can enter 2 times per year at a land border VISA EXEMPT (no visa). If you entered once visa exempt and once with a tourist visa, then you have not entered 2 times using visa exempt. You can still enter 1 time at a land border visa exempt and unlimited times with a visa. If you leave and return by air, given your history, I would expect you to have problems at the airport.