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.
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 …. ?