If you're new, starting a "network" isn't going to do anything good for you. Start with a single server to get players. The performance really goes by the price for those Kimsufi servers, so the choice depends on how much you want to pay. We can't tell you how many slots specific hardware can hold, it depends on many other factors.
I would also like to point out that the i5-3570S is a million times better for minecraft than the i7-920 provided on the internation pages.
KS-5 with Xeon 2 x E5504 and 16 GB ECC of Ram. Single Thread Rating: 831 / 13 = 64 players 8 Threads and 8 Threads for cpu (there are two) = 8 or 16 heavy servers? 16 gb - 2 (for the os)= 14 gb (avaible for minecraft) It's good?
Sadly unless you live in Canada, you cannot get that one. Since you need to give up an address, that is in Canada :S.
I don't seem to be able to login with my Kimsufi account registered under country the Netherlands, and when you want to register a new account your only option is Canada.
You'll probably want to run more than one server in that case. You *may* be able to squeeze them on one instance, but it really depends on how many players you plan to have on each.
I made up my mind. I need a server to host 1 hub and 10 others minigame servers... I'm starting a new network so i don't know how many slots i need...