The minimum memory requirement is 8 GB, but more memory might be necessary to handle peak demands. You can reduce the memory requirement for collecting from multiple vCenters (VC) in these ways:
For example, if you have three VCs and the largest VC manages 90 hosts, your "proxy number of hosts" is 117.
117 = 90 + 0.15 x 90 + 0.15 x 90
The number of ESX hosts above is guidance only, and is based on a typical virtualization environment with approximately 10 VMs and one datastore per ESX host server. More dense environments require larger memory sizes.