@Felicity ********
by land border crossing entries, Max of 2 per calendar year.
By air, no official limit...but you run the risk of being questioned by Thai Immigration Officer who greets you on arrival into Thailand. And possibly DENIED entry. As you are not considered a tourist.
In the future, get the METV multiple entry tourist visa in your home country before you fly to Thailand. With planning it can get you up to 9 months stay in Thailand with a few simple land border bounces.