I have 11 HomePods/HomePod Minis and it seems 10 is the max number I can Airplay to simultaneously. Even if the Minis are stereo paired, it still counts them as 2 devices. Still trying to find a workaround, but no success yet.

Lots of people seem to be seeing the dropoff issues, but it doesn't appear to be from the number of devices. For me, airplay from a MacBook, HomePods would drop out if I was also airplaying to older Apple TV's (using them as speaker nodes). One I stopped including the Apple TV's, the HomePods were not perfect, but much more reliable.

If you're airplaying from a Windows computer, do a search on Apple Discussions. Someone wrote a background utility that micro adjusts the volume setting every few minutes and that's fixed the dropout problem for them.