You can add snapshots to report history as follows;
1. Goto Report Manager==> Contents page
2. Then mouse hover on the item that you want to view history for and click drop-down arrow.
3 In drop-down menu, click View Report History.
4 Click New Snapshot.
5 Click Apply.
About maximum number of snapshot:
1. Open Report Manager, click Site Settings on the global toolbar.
2. Select 'Keep an unlimited number of snapshots in report history' if you want to keep all report history indefinitely. Otherwise, select 'Limit the copies of report history' to specify the maximum number of snapshots that can be kept for your report.