Hi there,
There is no hard limit, but there is a limit of 10000 EMS notifications waiting, and you need to assume also a concurrency limit for how many it will try to generate to get through a queue. And associated performance impacts of running so many EMS notifications each time an event is fired.
So basically, 10000 is way too many. 1000 is also probably way too many. Is 20? Unlikely unless the cluster is small and busy.