Yeah, I know it can get confusing because both maps and mods come in the form of .REZ files. But maps and mods are not the same thing. If you install maps as mods, they won't work. And vice-versa.
When it comes to how many maps and mods you can have . . . I'll try to explain it. (Thing is, the documentation that comes with REZ Handler explains all this. But, of course, nobody reads the manual.
With the Official patch, you can only have 6 map packs and mods active at the same time. That's a total of 6 .REZ files active.
With the Unofficial patch, you can have a total of 13 map and mod .REZ files active.
Notice I said "active". You can have as many map and mod files as you want, but only 6 or 13 active. REZ Handler won't allow you to make more active than the limit. But if you manually went into the folders and made more .REZ files active, then Tron 2.0 would start to have problems because you went over the limit.
Unlike maps, where you can have multiple map packs enabled at once, for mods you can only have one active at a time. Again, you can have as many mods installed as you want. But only one active at a time.
If you enable a mod, it takes one away from the number of map packs you can have active. So say your limit is 13. With all mods disabled, you can have 13 map packs active at once. With a mod enabled, you can only have 12 map packs active at once but the total number of "active" .REZ files is still 13. A total of 12 maps and 1 mod.
Well, I hope you understand what I wrote. I tried to keep it simple, but it's not easy to explain. It's all a bit complicated, actually. LOL. Monolith really didn't design the Jupiter game engine very well for Multiplayer.