The highest amount of profiles I've ever used in a mission is around 100 or so for each side, but that decays quickly because they are actively killing each other.
And by that I mean total profiles active in the mission itself (I use a profile counter for that), not the amount selected in the objective modules.
Depending on the battlefield size and amount of objectives available where you are letting profiles spawn (and the filtering you use in the objective modules), the amount of profiles for each side can fluctuate a lot. The more objectives available, the more profiles.
Example: Company 400 set to spawn all over Altis will yield more profiles than a Company 400 set to spawn all over Stratis.
What I like to do when testing is to turn debug on for the Virtual AI module and then watch. If the map feels nicely populated after about 30 minutes or so and performance seems ok, that's generally good enough for me.
If profiles die too fast, and large swaths of the map feel bare, I'll either increase the amount of profiles in the obj modules, lower the filtering, or add custom objective modules to fill in the gaps.
CQB can also fill in the spaces nicely as well. It can sometimes make a town with only one active profile feel much more populated and dangerous. I can't even tell you the amount of times I've debugged something thinking a city only had one profile, and then I debug in the tablet "mark active units" (or whatever it's called. It's an Admin action and I'm not at home) and then seen the city filled to the gills!
Sometimes profiles will make their way to the location after you get there too and yo can actively be fighting tooth and nail for 30 mins to an hour!
To slow down the speed, the virtual AI module has an option to lower the profile speed. I like 25% (the slowest) but 50% is a popular choice as well.
I'd say shoot for good performance first on your client, then work your way up and test. You'll find a nice balance, it just takes some tinkering and testing.
I'd also say try downloading some other people's missions too and take a peak. AuTigerGrad has a ton on Steam you can certainly learn from.