There is no limit on how many can be imported however too many in one modeldata (15 is a good number to stop at) will cause the game to lag so be careful when importing models.
Not always true. When using simple models (say, stage builder blocks) that are animated, I don't think it lags.
Say, Mewtwo2000's stage builder block stages.
BTW, I might make a guide for stage builder block stages.