you're possibly thinking of a multiple entry visa, each entry allows x days and you must exit the country by that time, you can then re-enter Thailand if the visa hasn't expired and you are stamped in again for x days. For non Imm visa it's 90 days stamped in, for METV tourist visa it's 60 days.
there is a multiple and single re-entry permit, there is a visa with multiple entry, no extensions come with reentry permits these are bought seperately from the extension, there is NO multi extension.