It's getting trendy to say "RBI" to represent the plural of "RBI" (e.g. "Hack Wilson had 190 RBI in his best year."). I'm not sure when it started. The first guy I can remember doing it was Keith Olbermann back in his SportsCenter days. Whoever started this needs to be spanked. The plural of RBI is RBIs.
In the English language, absent bizarre exceptions, we use the letter s to represent the plural. Bizarre exceptions include word reindeer, even though it always makes me smile when little kids say "reindeers." This convention extends to acronyms.
To refer to multiple attorneys general in the acronym form one says "AGs". ("The AGs of the Northeastern states will have a conference call to discuss interstate liquor sales"). To refer to multiple prisoners of war or weapons of mass destruction, one says "POWs" or "WMDs."
We are a smart-minded messages board based in a non-pretentious midwestern town. We have the sense to know that RBI are not reindeer. So I urge you to resist this annoying trend to use "RBI" to represent the plural form.