On the "Prune&GC" tab, you can create pruning jobs, e.g. to keep only the last 5 backups.
If you are not quite sure whether the rules really do what they are supposed to, you can test them first with the Prune simulator.
For further information please have a look at [0].
[0]
https://pbs.proxmox.com/docs/maintenance.html#pruning