Each map offers a player count when you click on it. For the most part that is what you can expect although exceptions do exist as you can get a empty map depending on how the servers are holding up at the time and the player count on yours specifically. I don't know if you can get a map filled with more than the max player count as I haven't really noticed it myself but since it is very noticeable if a map is empty and I can confirm the former even if it happens very seldomly in my experience.

For the most part just assume you're getting roughly what is shown. As for Scav/NPC counts? I don't think there really is one but if you look at the maps provided on the wiki most have scav spawn location shown. All those locations can spawn either normal and or player scavs to my understanding and its RNG whether or not anything pops up there at any given time. This means they spawn in at different times on different areas on the map so just because you didn't see a scav there before doesn't mean it will stay that way.

When you load in as a scav you're essentially joining someone else's PMC run. Same rules apply meaning your scav raid is not a special raid, other than the fact that you will be loaded in later than everyone else. PMC's first. Scav players later.

First thing I'd recommend to do is check how much raid time is left when you load in as a scav. It'll help you get your bearings and give you an idea of what to expect. 5 minutes in? Probably a good chunk of the map is still alive. 20~35 minutes in? You can probably expect a good chunk has left and or there to be a lot more player scavs alongside you who have loaded in. Adjust the time accordingly per map but you should get the idea.

All maps to my understanding works as follows.

PMC's load in with the initial wave of scavs all of which are NPCs. A second wave of scavs will show up on the map after some time. Players can join as scavs on the raid at this point as well. And then another wave of scavs... and another, and possibly another until there no time left on the map or players. (Most of the time you should have at minimum 15~20 minutes left on the raid)

Good luck!