Do you mean "live snapshot"? If so, there is no possibility to limit them now.
The other replies in this thread were referring to "number of backups sets" that proxmox keeps (i.e. if you set it to 3, you will have the last 3 backups of the vm, next backup the oldest in the set will be replaced with the newest etc.), the fact that you do the backup as snapshot or not is irrilevant and not related with "live snapshot"