How many times can you do a border run to obtain a visa stamp in Thailand, and can I get a visa in Koh Samui?

hello all im looking for some advice , how many times can you do a board run to get a visa stamp , also can you go to the office in koh samui to get one also before you must do a visa run .

im looking at a single entry visa at £25 for 60 days

or should i get a multi entry at £125

Any advice would be most appreciated .

Im just a little confused on what to do