Can I extend a 30-day visa exemption more than twice a year in Thailand?

2 only by land. No limit by air

No it's not correct.

You are limited to entering Thailand at a land border without a visa (visa exempt) only 2 times per calendar year. There's is no written limit to how many times you can arrive by air.

But you're not going to live in Thailand on visa exempt or tourist entries because after 2 or 3 in a row you're going to be pulled aside at the airport and asked why you're trying to live in Thailand without a long term visa.

Each tourist entry (visa exempt or tourist visa) is eligible for one 30 day extension. So if you're able to get in, you can apply for the extension every time.