that's a kind of visa. It's called multi entry and allows you to get inside the country up to 6 months without any extension (which would provide 3 more months). That's not abusing the system. Getting in the country on visa exempt for multiple times is
there's not a fixed number. Do you stay in thailand less than 180days per year? then it's likely to have no issues. Do you spend in thailand long periods of time using tourist visas and visa exempted? then you might have issues.