@Christopher ********
you are confusing your terminology. a visa on arrival is a visa SOLD to people from 18 countries when they show up here for 2000baht and stamps them in for 15 days
You are talking about a "visa exempt entry" <- that's where you show up without having bought a visa from a thai consulate before you came here and you get stamped in for free without a visa (hence the catchy name "visa exempt" 😛 ) for 30 days.
You can have TWO 30 day visa exempt entries by land in a calendar year and there is no official rule on how many you can have by air entry.