You can get as many single re-entry permits as you want. The downside is it takes a lot of space in your passport and if more than 3 single re-entry permits a multiple re-entry would have been cheaper.
I often take a multiple re-entry permit for the conveniance.
baht must be certified by your national embassy in Thailand. Some countries do not give a income statement such as USA, UK etc. Then only 800.000 baht on a Thai bank will be accepted.