When you schedule a backup job you can specify how many copies or how old can the be if you have the Datastore.Prune permission. Other than that you could go to the Proxmox Backup server UI and sort backups according to creation date in their datastore.
Yes, using the web interface you would have to go through each one. Alternatively, you could use proxmox-backup-manager to manage them and create a small script if needed.