Hi,
For vSphere 4.1,
Hosts per vCenter Server 1000
Powered on virtual machines per vCenter Server 10000
Registered virtual machines per vCenter Server 15000
Please check http://www.vmware.com/pdf/vsphere4/r41/vsp_41_config_max.pdf
For licensing, Goto Home -> Licensing -> Manage vSphere Licenses -> Add the license keys and follow the instructions.