Good morning.  What version of vCenter are  you using?

vsphere client version 7.0.3

Difficult to calculate exactly as it depends on a lot of environmental factors - like how many disk each VM has, size of VMs, what your RPO is etc.

HotAdd requires SCSI slots, and a maximum of 60 disks can be mounted at once if you have 4x scsi controllers on the HotAdd proxy. Each disk on a VM will use a scsi slot during backup - although performance of the proxy is usually exhausted long before the 60 slot limit is reached.

In either case, there is a guide here on sizing proxies vs front end terabytes (i.e how much VM data you need to protect)

https://documentation.commvault.com/2022e/expert/32813_vsa_hardware_requirements.html