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You can enter as many times as you like. There is no limit. There is only one rule. To "reset" the time that you get, you can go anywhere except Canada, Mexico or any of the adjacent islands to the U.S. If you go to one of those, to "reset" the time, you have to be there 30 days or more.
Now that is the only rule. However, at some point, if you keep doing that, you will be stopped because you are not supposed to use a tourist visa if you aren't really a tourist and it looks like you are living in the U.S. and just leaving to reset the time. So even though it is not a rule, the general idea is that you should spend more time outside of the U.S. than you do inside. So if you come for a month, you should stay out for 3. If you come for 6 months, you should stay out for a year. As I said, this is not a rule, just a general idea so that the officer doesn't think you are living in the U.S.
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