There is no limit documented, but that doesn't mean there isn't one (I would bet that there is). You could probably find it by trial and error.
I would question the wisdom of relying on hundreds (thousands?) of find-and-replaces in a load rule. There is no way to maintain the listing except manually, and no way to audit it short of "printing" the load rule.
Discussion of similar issues in this recent thread: Saving rule file in Essbase
I have used many hundred replacements in a load rule but do not recommend it for the reasons Tim gave. If there is any other way (ODI, relational table lookup, FDMEE) I would use those instead. It would be much more auditable and maintainable. A while back I requested the ability to import and export or have external replacement lists in load rules, but I think the development team has bigger fish to fry