How much water does a diapham or bladder tank should hold for my well

Depends on the size of the tank. It will not hold as much water as the tank size, so that you will not have 40 gallon tank holding 40 gallons of water. This brochure has a graph towards the end which will prove helpful to you.

http://www.freshwatersystems.com/specifications/Well-X-TrolBrochure.pdf

how much water does a diapham or bladder tank should hold for my well n Why are you asking?
How large is your tank and what's the tank pressure set at.
If you pop the snifter valve on top of the tank and you get water then the bladder's ruptured and the tank needs to be replaced. Let's have more details. Cheers, Tom