Symptoms:
You can use the vCenter Server Appliance Management Interface to back up the vCenter Server instance, however the option
Number of backups to retain \ retain last X backups wouldn't work. This is applicable for all vCenter version 6.7.X with all available protocol type (FTP, FTPS, HTTP, HTTPS, SFTP, NFS, or SMB).
You may see number of backups higher than the retention period given in the VAMI backup schedule.