there is no official amount of times on air border crossings… 3-4 is not a realistic number as I know people that use Bangkok as a hub and regularly travel every week Out/In…now with that being said eventually you will be questioned but not at a certain number of entries or it could be that you are never questioned…
you can enter then extend the visa exemption stamp at your local immigration office for a cost of 1,900THB, each immigration office has their own criteria on when you can extend it. some don't allow you to extend until a certain day count to expiration others allow you to extend It the day you land....
for land border crossings you get 2 per calendar year 01 JAN - 31 DEC.....
airlines are the ones that care about RT tickets due to the economics of it if you are denied entrance
You can either pay the overstay fine ฿500 per day…..or….go to your local immigration office and pay ฿1,900 for a 30 day extension…if in Bangkok you can do it the save day as you land as long as you land when immigration is open and you can make it there before it closes…good luck