It is not always about how much RAM you got. I could have 8 GB of RAM, but it won't mean anything unless I have a upload and download rate that can support those players. Both are an important factor, each one can only tell you so much, so whichever is worse is what you should assume your max is, until you get testing done and can see if it is so. I have a dedi and I have seen others of the same size run 7 people no problem with 1 GB of RAM, but they have a really good upload rate, they claim 100 Mbps, so what would slow me down is the RAM in this case. You will need to take your internet speed, whether from your own internet or a service, to answer this fully.
5 people per 256 MB of RAM seems to work pretty well, but it won't support that many if you're doing any flying, world editing, animal spawning, etc. You might get some lag also. To be safe, I'd say 7-8 people per 512 MB of RAM, maximum of 10.