How many strikes does it take to be sent to jail when you're on probation?

Any violation of probation, ANY, can send you back to jail.

This answer is absolutely right. More right than your expectations about probation. Even though it is commonly granted, it is a gift. A lot of what may result from a violation depends upon your criminal record, the attitude of the judge and prosecutor, and the nature of the violation - whether you failed to notify the probation department of a change in address vs. a positive drug test vs. committing a new criminal offense. If you commit a new crime, you are not entitled to bail, and in most, if not all states, there will be extra time added to the regular statutory sentence.