The reason for the limit is that Skyrim (and other Bethsoft games) uses an internal code (the Form IDs) for everything added by a mod. This also does include the expansions, and any such mods downloaded through Steamworks rather than Nexus. not mods that simply change models or textures).
esp file so they can change actual data in-game (i.e.
This only counts certain types of mods, though specifically, ones that add a. The absolute upper-limit is 253 total mods.