... if I start a Backup Job with 20 VMs all 8 tasks are used in parallel? Let's say: The first and second VM in the job contain 5 hard disks each. Are all 5 disks from the first VM + the first 3 from the second VM being backed up in parallel?
... another Job cannot start until there are free task slots available again?
What happens if a second job is scheduled to start at 03:00am? Will it be put on hold until free Tasks slots are available again? Which means it could start a lot later than intended I guess?
Yes, it will be put on hold until Job1 starts its last disks and there are free slots left. Yes, that might delay the start of the 2nd job.
How could this be solved?
A) Add a separate proxy
B) Increase the amount of slots
C) Use the backup window
D) Use the "multiple repo" approach that you've described (it will cause the proxy to underperform though)