Just back from Mae Sai immigration. According to them, there is no limit on the number of times you may cross the border and come back on a 30 day stay without a visa. However, they did say that if you are considered to be going very frequently, they will notify your embassy who will come interview you. In Mae Sai, this is due to the drug trade and I don't know if it is the same elsewhere. So if this 6 time rule does exist, it doesn't seem that it is widely enforced  by the immigration officials. Indeed, they don't even seem aware of it.

Mai Sai Immigration seen to be a law unto themselves.

They make the rules up to suit themselves

I think Mae Sai Immigration is exagerating.  There is no way they can influence an embassy to do anything.  Sure they can notify, but it's the embassy's decision on whether or not to interview you.  I know several farangs who make monthly visa runs up there and not one has ever been interviewed by his embassy.

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I have to agree with mesquite. Although your name does go into a computer each time you leave so theoretically at some point an alarm might be triggered. But as Bob said, this is set up to catch drug runners. And a farang stupid enough to run drugs over the Mae Sai border deserves to be caught anyway.