Hi,
Best practices is please delete the snapshot one by one and do it one VM at time.
When using Delete All in the Snapshot Manager, the snapshot furthest from the base disk is committed to its parent, causing that parent snapshot to grow. When that commit is complete, that snapshot is removed and the process starts over on the newly updated snapshot to its parent. This continues until every snapshot has been committed. This can lead to an aggressive use of additional disk space if the snapshots are large. Use care when exercising this option if there is not much space available on the datastore.
Can you give me the screenshot of datastore, so that we can find how much size occupying for snapshots. also let us know VMDK is thin or thick provisioned.
Note : Please do it this actvity on non-working hours.