Limit of 15 routes in module.routing.yml
It's just you I'm afraid.

Take the system module's routing file for example; there are a few dozen routes in there, plenty more than 15.

Symfony doesn't impose any limits either, so I'd check that your YAML code is properly formatted and valid. If you're positive it's all good, you've probably hit a bug and the best course of action would be to report it.