I am making a mod with a new worldspace and if I put too many NPCs in certain cells the Geck crashes


Any help will be appreciated

How many are we talking, exactly? It wouldn't surprise me if either the GECK or the game itself had a functional limit on something like that. If, for example, you were trying to stuff 50 people in a shop that would simply be the likely result.
On that note, are the cells you're noticing this crash in smaller than the others it does not occur in?