You can always get a brief overview of the Fleet Status of any of your fleets from the Fleet Management page.
Click on the "Vessels" tab and select a fleet of yours.
All vessels that constitute the selected fleet will be listed below. At the bottom right corner of the list, you can see an indication regarding the total number of vessels in the subject fleet.
You are also able to use the Fleet Status Controls in order to dictate each vessel's status. You may, also, use the "De-activate all vessels" button to set all vessels to OFF at once.
Tip: Click on any vessel's name to get to the Vessel Details page of her.