You can set the number of Versions to keep in Retention Settings. Version is the number of copies to keep of each file. Does that work for you? Instead of 2 NAS drives, have you considered using a Hybrid backup that would keep one copy local on the NAS and create a second copy in the public cloud of your choice for disaster recovery?

I should have mentioned, this is an image backup, not files-based backup. We are doing 2 NAS drives in addition to the cloud for extra redundancy. I see what you are saying about the versions but we were under the impression that versions with image-based backup is not the number of image backups but versions of the files within the backups. Maybe I am wrong, which would be a good thing. Thanks!

It's the number of image backups and is not file-based.

Great, thanks!

Have you considered leaving one NAS onsite and mirroring the backups to the second NAS daily before taking it off-site? That way, you'd have a complete backup set in two places. The risk you have with rotating drives is you may not have current backups on both, and depending on schedules, you may end up with a split set, making restore impossible without access to both drives. You could use a built-in NAS utility to copy to the second drive or using something like Windows Robocopy to facilitate the mirroring of the NAS to the second.