Because the provisioned space for thin disks can be greater than the committed space, a datastore over-subscription can occur, which results in the total provisioned space for the virtual machine disks on the datastore being greater than the actual capacity.
Over-subscription can be possible because usually not all virtual machines with thin disks need the entire provisioned datastore space simultaneously. However, if you want to avoid over-subscribing the datastore, you can set up an alarm that notifies you when the provisioned space reaches a certain threshold.
For information on setting alarms, see the vCenter Server and Host Management documentation.
If your virtual machines require more space, the datastore space is allocated on a first come first served basis. When the datastore runs out of space, you can add more physical storage and increase the datastore.