This is a quote from Kevin Michaluk on crackberry.com when he did a "stress test" with BBM:
Yup, there's a limit! Though I still can't say what the exact "max #" of contacts is. This message finally popped up when I hit 8007 open Requests, 1388 actual approved Contacts, 616 Pending contacts, and 317 active Chats (see screencap below) and then tried to approve another request.
He goes on to say that the limit of approved contacts without any pending requests should reach about 2000. The full article is here: BlackBerry Messenger 5.0 Stress Test Update | CrackBerry.com