Personally I don't think there is a limit as long as each patch is applied properly. Maybe when they are overlapping all over the shop as I don't know if the vulcanising solution works as well on the top of the patches.
I've certainly had some in use which looked like they had measles.
Had 5+ on my last tubes, didn't have a problem. Now one has split at the valve and I think that is a tube killer because the patch won't adhere to the metal valve.
How many patches per inner tube is deemed o.k. to keep using … asking for my very best friend - even if he is a cheapskate twat …. ?