it is turn based (it is similar to chess, but quicker). I expect 2request/second per user.
Additionally, there will be a lobby with all rooms and room variables updated for every user in a rate 20 updates/second. These are just the estimates, but i would rather have more powerfull server then less poverful laggy one
A mid-range server should be able to handle 20K+ user with no effort. Let's say a Xeon class 4-6 core machine with 4-8GB RAM should do the trick. If you're also planning to run other services (database, web server etc...) you may need to evaluate the impact of those. For 20K users I'd rather use two dedicated machines, one for SFS2X and the other for the DB/HTTP