I remember a long time ago I was spending a lot of time searching for this data, but couldn't find it anywhere (at least not in a precise form, more like "yeah 100MBIT worked for our X players session). However now when I have more experience playing on low-end connections with low player counts, I think I have this more or less figured out.
Keep in mind that everything here comes from seeing how many players can play smoothly vs how many players need to log in in order to clear see desync issues - Never actually looking at ingoing/outgoing data (can't do it anyway on a hosted server as far as I know).
One thing I noticed is that the amount of AI doesn't seem to have a noticeable effect on how many players I can get on the server without problems.
My conclusion so far is that you need something around 0.1MBIT per player. How?
All upload speeds were measured by speedtest.net
My connection: 0.2~0.23 MBIT upload. Maxmium players other than myself: 2, with perfectly smooth play. If 3 players log in, significant lag occurs and it becomes almost unplayable.
A friend's connection: Not sure whether it was 0.4 or 0.5 MBIT upload, but max players that could connect and play smoothly was 4 (other than the host), 5th player connecting would cause obvious lag.
Another one who I think (but not totally sure) had 0.8MB could have up to 7-8 players connect (again don't exactly remember, this was very long time ago, and I usually don't have that many players connecting).
If anyone has more data that could give us more accurate estimations of Arma 2's exact bandwidth requirements, please post it and I will try add it to the first post if it provides new information that everyone should have easy access to. It would especially be interesting to see if with more players we would see a non-linear increase in bandwidth requirements, as can be suspected when you look at the results I got.