Officially, visa runs by air are unlimited. But if you do too many the Immigration officer can suspect you of working and deny entry. The first 6 months should be no problem though.
If your going to stay more than 30 days, get a 60 day tourist visa from the Thai embassy nearest you in the US before you come. Going to immigration in Thailand for an extension is a pain, especially if your coming high season, Nov-March, it will be very crowded.
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