I've got a server with about 75 AJAX client users. Right now, top reports 287 processes, so those numbers don't seem out of whack.
What kind of processor does this system have? It looks like whatever you have can't handle all your users. Are your 5K users IMAP/POP based? Do many use the webclient?
SSL will increase your CPU load more than it will your RAM usage. That's your major concern here, it seems. You might need more horsepower if you're seeing loads reaching 10+. Are there any speed problems right now?
If you want to see RAM usage, you should run "free" instead of top. It's arranged a little better, and it's easier to see how much of your RAM is just sitting in buffers/cache (not really being used, but Linux still has ahold of it and will use it when it needs it)
Code:
total used free shared buffers cached
Mem: 16432296 16268328 163968 0 602404 7497476
-/+ buffers/cache: 8168448 8263848
Swap: 2031608 172 2031436
Mine shows about 8G of usage, there's another 8G just sitting around waiting to be used.