Can anyone comment on how many instance of VDP 6.1 that needs to backup a total of 50 to 80 VM???
Basically, the answer here is zero. Reason being that VDP went End of Availability over a year ago and is essentially a dead product. So you shouldn't be using it at this time if you don't already have it deployed.
It's 8 concurrent VM backups. If my memory of this legacy product serves right, then if you have Job1 with 10 VMs and Job2 with 4 VMs and you happen to kick off both the jobs, 8 would run and the rest would be Waiting - queued
Progress in GUI tends to hang at 92 percent. And you can monitor it by using mccli activity show --active