It depends on the plan that you sign up for.
If you sign up for the $35.99, there is 40-50 stores per server, while the higher plans like the $129.99/mo has 10-15 stores per server.

There are other plans as well like the semi-dedicated (4-5 stores per server) then the dedicated option (1 store per server). It all depends on how much bandwidth and space you need. You can even grow to multiple dedicated servers if you need to (load balancing/cluster setup).