According to an article I found, Morricone had a minimum five-film relationship with no fewer than 28 directors. I haven’t checked it but I wouldn’t be surprised.
Mauro Bolognini leads the way with 15 films, which I’d say is probably an underrated relationship, given that the ones that immediately spring to mind are Leone (6) and Tornatore (13). If you think about it a bit more, you may alight on Pasolini (8), Corbucci (7) and Argento (5).
After Leone, however, my favourite might just be Elio Petri, with seven films in common with Ennio. Definitely underrated, in my opinion.
From relatively highbrow Italian films to the ridiculous, Gerald Thomas and Eric Rogers worked together on 22 Carry On films (if you include “Don’t Lose Your Head” and “Follow That Camel”; otherwise 20).