You could probably have 100+ slots with 500 MB of RAM. The problem is there aren't any maps that can support that many players properly. I would say 32 slots is the max any server should have. If you want more start a second server.
Here is a .sh script for starting a dp server:
#!/bin/bash
cd /home/user/servers/normal/paintball2
while true
do
echo "Starting paintball server"
./paintball2 +set dedicated 1 +set port 27910 +set maxclients 20 +exec myserver.cfg +map shock
sleep 10
done
Just make sure to change the directory and the name of the config file.
If you have any other questions you can ask me on IRC.
www.SupermansPub.com/chat.html