I know of no limit on the number of times copper can be reheated. From a practical standpoint, if the problem area is in a difficult place to access, you might want to consider cutting the problem area out and replacing with new. It would eliminate cleaning the old solder off. You would need to use a coupler on each end of the repair. Do you know why you had a problem? The most common problems are not cleaning the fitting and pipe end, not fluxing, poor heating and solder application, moving the joint before it was cooled and set, and running water into a hot joint to try to cool it faster.