As Alexander already said, the amount of data to backup is the same, with or without tags, unless you leverage tags also to use differenr RPOs in the environment by creating more frequent backups. Performance are not affected by the number of VMs per job, but the speed of the different components, and the numbers of available processing slots in the proxies.
For the size, in addition to parallel processing helping to spread the load to all the available proxies, I can also recommend per-vm backup chains (http://www.virtualtothecore.com/en/veea ... up-chains/
). With it, you can be assured that no job will ever grow in size regardless the number of VMs in it, since each VM will be stored in a different file. So, having 10 or 500 VMs in the same job will not matter.