The research is apparently not at all easy and one of those potential avenues for historical research where the sheer backbreaking tedious labour scares people off (?much like counting naval numbers?) - there were no central figures collated until relatively recently and even the annual directories on which this is based aren't absolutely accurate and aren't designed for the purpose, their primary purpose is as a record of addresses and nowadays phone numbers of the people you might need to contact. To do it properly you have to go back to each of the registers in each diocese (and monastery or order) and count, and cross-refer. There's been some research on earlier periods published by the Catholic Record Society but their stuff isn't on line (the journal is only going on line this year, it's just moved to OUP or CUP)