If you're having instructor changes it's important that the school keep good training records. That way a new instructor can review what you've covered in order to minimise duplication. If the school has it well sewn up it need not be a problem.
I had over 10 instructors during my PPL course, and I still passed my skills test at 43hrs. I'd say consistency is far more crucial than the number of instructors. If you're not getting up regularly you'll be covering old ground because you need to, not just because you've got a new instructor.