Cop is an acronym. You can pronoun it. It's okay to pluralize acronyms. RBI is not. To prove this minor point, people say ribbies to make the abbreviation, RBI, into an acronym. Other acronyms are scuba, radar, etc.
It's only a minor point, but RBI doesn't really get an "s" on the end to make it plural. It's not an acronym.
I wasn't an English major, but I was a journalism major.
This whole thing is so stupid. It's RBIs in my view. I would argue that RBI is now, through usage, a word in and of itself, and should have the 's' at the end in the plural usage. The purists out there who insist on saying 'three RBI' should then start saying that 4 'cop' responded to the emergency call, since cop is an acronym (for 'constable on partol'). Of course, no one does that because, through usage, cop is itself considered a noun on its own.