On a tourist visa, is there a limit to the number of visa runs you can do, to keep visa o.k. Thx.

You do not make visa runs on a tourist visa. It is either a one, two or three entry permitted and each entry allows a 60 day stay. That stay can then be extended for 30 days at an immigration upon payment of 1,900 baht and filling out a TM.7 and providing photo and reason for staying longer. Many consulates do not look kindly on issuing a new tourist visa if you appear to be living in Thailand and can not provide justification for more tourist travel.

If you are thinking about the visa free entry of 30 days there is no set limit but some exit points have not been happy with more than 2 done without a break outside of Thailand. But this varies with place and time.