1) How many datacenters can we create in Vcenter server 4
2)How many number of host can we create in Vcenter server 4?
3)How many Vm's can we create in Host (ESX4)?
Configuration Maximums should answer your questions.
3)How many Vm's can we create in Host (ESX4)?
It depends on the memory usage and workload, how many VM's can run on a host simultaneously.
André