It's nothing to do with number of entries. I know several FIFO workers who enter Thailand every three weeks on visa-exempt. However they stay only a week, and then they're gone again. Those they are looking for are those who are trying to live in Thailand on a series of tourism stamps. Eventually you'll be denied entry, and get a black mark on your immigration record. Advice is simple:- Get a proper visa
Yes that's true, but it's not that simple. As I've explained in my response to your other post, DTAs play a huge part in whether expats are actually tax resident in Thailand. There are way too many variables to be able to categorically state someone is "liable" for taxation in Thailand.