Average size of main movie titles with one HD track varies between 20GB-to-30GB.
35 movies would be a realistic number for uncompressed 1 title backups.
Depending on the authoring and thus the file size, the number could go up or down.
A lot depends on how you rip them. If you rip the entire disc, then there is no way to estimate since they vary so muck with the special features, directors/ heater/extended cuts, etc... However, if you use clown BD and just rip the movie, then it's a little easier. I always estimate 25gb per BD (movie only). Hope this helps.