There are different types of mods.

• "Direct" mods directly modify game files. In theory, many direct mods can be used in tandem if the files are carefully modified to include many different changes. Once installed, these mods are always active by default. One or more modders calls these "injection" mods (but I don't personally like the term because it can mean something else entirely).
  
• "Dropdown" mods are the type you seem to be referring to. You select one in the game lobby before starting a game. They can't be used in CtW campaigns and are not active by default. Because only one can be selected, they can't be combined unless a new mod is created that merges the changes from multiple mods together.
  
• "Local" mods are halfway in-between. Files are placed into the /mods/ folder and when the game is launched, it loads these files instead of its original files. "Workshop" mods work similarly, but instead of the files being in the /mods/ folder, they exist in the Steam Workshop folder. These mods are supposed to be able to be toggled on and off through the in-game mod manager, but unfortunately often don't fully load or unload due to bugs / flaws in the process. It can be possible to have multiple local mods together so long as they modify different files. Local/Workshop mods are also not compatible with direct mods that modify the same files.

If you use mods other than dropdown mods, you can use more than one if you do it carefully. For example, you could have one mod that replaces the spy bribe sound, another that makes all citizens wear top hats, and another that allows you to build 6 farms per city instead of 5.

However, if you're only using dropdown mods then you're unfortunately stuck with a single mod unless you manually (and carefully!) combine multiple mods together. You also will not be able to use them in CtW campaigns.

Disappointing, but thoroughly explained. Thank you so much! Looks like I'm settling for single loads!