there's no such thing as "max tourist stay". Whether your plan is feasible or not will lie solely at the discretion of the immigration officer upon your return to Thailand.
It may be or it may not be. It depends on the immigration officer who sees you and whether he or she thinks you're trying to live in Thailand on a visa exemption.
you used to be able to do it on their website, I did it on my laptop. I'm assuming you still can but I haven't done one for nearly a year now so don't know for certain if that option is still available.