There is no hard limit to the number of sims that can be in a household when using NRaas mods. If you have more than the 24 that PortraitPanel can possibly fit on one screen, the space bar can be tapped to advance through them into the "next page" of thumbnails and back. However, after a certain point, and this is difficult to define as it will be different for players on different system hardware and will depend on how busy the game engine is with the world being played, game performance will suffer and potentially grind to a halt.
Personally I don't find it fun to play households greater than 15 or so (perhaps not counting pets) because it's just too many sims to try to keep track of all at once. But that's just me, other players reach their tipping points in different places. And it also can make a difference as to whether this household is the actively being played one or not as to how it behaves -- if it is ever made inactive even for a moment, the game will stand to dissolve such a household unless NRaas StoryProgression is in place and set allow one of such a size.
The serious error on startup usually means that a game file is being held open by some other power when the game tries to load it. Sometimes it's enough to have portions of the TS3 user game folder open in Windows' File Explorer, others have had issues with anti-virus and backup programs getting in the way. Reported a bit less often, it could be a corrupt save or one that has an unexpected character in its name. Do you have any other saves featuring this household that you can try to load?
For future reference, a pretty good place to bring such questions is to us at NRaas. It's kind of what we are there for. :)