Do I need a visa to stay 90 days in Thailand, or can I extend my 60 day stamp?

If you want to stay 90 days in Thailand, you will have to get a single entry tourist visa that lets you stay for 60 days, and then get a 30 day extension of stay based on tourism.

If you enter without a visa, you will be stamped in for 45 days, and you can get one 30 day extension of stay based on tourism, letting you stay for 75 days