This information isn't well known and unfortunately they don't tell you until you interview there how many they reserve for their own. Historically, I remember SUNY Downstate is notorious for reserving 3 (out of 6) of their spots for in house IM, and even favor the Brookdale graduates. It generally doesn't look great for the IM program when the Fellowship doesn't take at least one of their own.
Another pattern I had noticed is Community GI programs tend to favor 1 spot for an internal IM candidate. The risk with this is you have to hope you are the only one in that Community IM class who wants to do GI. If you are competing against a co-resident, a chief, or an Alumni in that Community IM program, you may need to apply very broadly.