I came across a poorly designed app that had about 60 1-1 relationships. I tried to enforce RI and Jet choked at some point less than that. I don't remember exactly when it refused to create any more relationships. I had to convert the db to SQL server since I felt that RI was necessary. So, that tells you that this is not a customizable limit.
I have never exceeded the maximum in any properly designed db. Even very large ones.
