I checked an 1P has 55 backup zips on my Mac. Any reason why I can't manually delete 54 of them?
1Password Version: 7.6Extension Version: Not ProvidedOS Version: Not ProvidedSync Type: Not Provided
If you feel comfortable with having only one (presumably the most recent) backup, then I suppose there's no reason. That said, unless I was desperate for space I wouldn't be inclined to delete them if I didn't have to. In general I'd rather have more backup than less.
One further question though: Are you using standalone vaults or are you using a 1Password membership?
You can certainly manually delete old backups if you want, there is nothing stopping you from doing that Is that not working for you?