Take look here: Number of devices monitored on Icinga
In most environments the bottleneck will be the load from the checkcommands, so you will never get the correct answer for your environment. On plain Icinga with agents (with no heavy checks) you may find an answer in this post: Icinga2 at large scale
Edit: Maybe its a good idea to pin some benchmark/performacnce posts in an overview thread, as this questions popping up regulary. I only can remember the two i linked here 