There is a Capacity Planning for Backup Repositories
report which shows number of days left for each repository.
If you want use the repositories altogether, I`d suggest to combine them into Scale-Out Backup Repository
As for Veeam ONE, we may include the "Total" information about repositories into the report, but that may be deceptive if we say, "You have 400GB left among your repositories", which in practice can be 100GB of free space on 4 repos and despite you have 400GB of free space, it will not fit even 150 GB backup file.
What do you think?