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.
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.
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.
More Information