A drobo is like a regular external hard drive in that it can be split up into smaller volumes or partitions. Just because you have 20TB of storage doesn’t mean you actually want a 20TB volume.
I recommend your volume size be based on the size external drive you are using for backup, so that each volume can be easily backed up to a single external drive.
For example, if you are using 8TB drives for backup, you might want to make 6TB volumes on your drobo. You want the backup to be bigger than the “master” volume on the drobo because backup software can allow you to keep snapshots of old data that allows you to go back in time if you accidentally delete , erase, or overwrite a file, or if it becomes corrupted. The more extra storage on your backup drive, the further “back in time” you can go (no DeLorean or flux capacitor needed.)
This means you’ll likely have multiple volumes/partitions on your drobo, and you should consider how you use them.
I recommend separating “hot” or frequently changed data from “cold” or seldom accessed data. Hot data is things you are regularly using, like your lightroom catalog, your latest photo shoots, etc. Cold might be where you sort older shoots that you are not accessing, as well as files that don’t need frequent modification. For example “2019 Raw Captures” might get it’s own volume.
Really cold data could be moved to an external drive to free up space on the drobo.
There are lots of ways to play this, but I’ll leave those details up to you.