Maximum Snapshots parameter
The parameter on the Snapshot Options dialog box sets the maximum number of Instant Recovery snapshots to be retained at one time. This parameter is available only for certain snapshot methods. Unlike the pane, it cannot specify which devices to use or the order of their use. (The parameter and the Snapshot Resources pane are mutually exclusive.)
When the maximum is reached, the next snapshot causes the oldest job-complete snapshot to be deleted.
A snapshot job is considered to be complete once all its configured dependent copies ( for example, Backup from Snapshot, Index, Replication) are complete.
Note:
For Windows clients using the VSS method on disk arrays that are configured for clones or mirrors: you must synchronize the clones or mirrors with their source before you run the backup.
For Instant Recovery backups, it is good practice to set the backup retention level to infinite. A retention period that is too short can interfere with maintaining a maximum number of snapshots for restore.