Hi, I am new to vCenter Server Appliance, I wonder the maximum of vCenter Server Appliance 5.1 update 2.
Browse down to this KB article: VMware KB: Minimum requirements for the VMware vCenter Server 5.x Appliance
In vSphere 5.0/5.1, the embedded database provided with the vCenter Server Appliance supports an inventory with a maximum of 5 hosts and 50 virtual machines. The embedded database is not configured to manage an inventory that contains more than 5 hosts and 50 virtual machines.
You should consider deploying the 5.5 VCSA instead (supports 100 Hosts using the embedded Postgres DB). I'm assuming you're not using a remote Oracle database (which supports up to 1000 hosts). With the embedded DB that comes with the VCSA, the advice is to increase the VCSA HD size to 120GB minimum for big deployments. Do this as a follow up step immediately following deployment of the VCSA so you don't have to worry about it later.
Check the release notes, related KBs, and config max guides for additional info if desired: