If a reallocation event happens once, and never again, it's not a concern. If it repeats again, it's time to pay attention. 3rd event, plan a replacement. If you get several reallocation events back to back, prepare to lose the drive, probably sooner rather than later.
The number of sectors is a factor, but the rate of increase is what is most concerning. Some drives reallocate a dozen or so sectors, then stay quiet for years.