1 mod active is the limit, yes. Modders are planning on gathering and creating 'Mod Packs' in the future which will combine seperate mods together.

As I understand it that restriction only applies to the host. Joining clients seem to autoselect the appropriate mod if it is installed.

I thought the game allows multiple addons to be select at once, but for some reason modders seem to class their map packs as full blown mods...

Due to the way maps are handled they must be classified as 'mods' and not 'addons'. Limitation of the system or something. I'm not a modder so I can't give specifics, feel free to join the Discord where a lot of actual modders reside if you want more information.

Devs fix plz

It currently works this way as an effort to prevent bad asset throws / desync a game. If it DIDNT work this way it would be VERY easy for someone to accidentally or maliciously modify files, load them up and join a game that did NOT have those same files loaded up. It sucks, but that is the rationale for it.

I might have a solution for this. The multiplayer host should be able to enable or disable mods for that multiplayer game from the create game menu. This way the players could discuss and agree upon which available mods to use. Even better a good idea would be a button to show the mods workshop pages in the shift+tab menu so that player joining can download them if they don't already have them.

Someone correct me if I'm wrong, but the game already swaps to or downloads a mod if you join a match without it active.

Yup, but it does not actually subscribe you to the mod. Had a few issues that way where mods had updated assets.

A single "config" mod can load many asset packages (though there is a limit). That will allow us to get mods working together in the near future. The mods just have to actually be set up with that in mind.