Hello @Gili Sela 

You can backup from 10,000 VMs to 18,000 VMs using single VBR, number of vCenters doesn’t matter. For more information:

https://bp.veeam.com/vbr/2_Design_Structures/D_enterprise_design/enterprise_design.html

Regarding proxies, it depend on many factors, however as a golden rule you can have at least 2 virtual proxies on each cluster.

For vSAN, it’s recommended to have proxy on each node.

Above for hotadd backup mode.

For direct SAN it will be different, check the below:

https://helpcenter.veeam.com/docs/backup/vsphere/backup_proxy.html?ver=120

thank you for the answer 

As noted number of VCs is not a problem but be sure to read the link noted.

you are welcome!