This depends on the grain amount in lbs. and the water to grain ratio that you are using in the mash. The grains absorb a certain amount of water, so you lose some there. Normally you'll want at least 5.5 gallons ready to go and to be on the safe side you could use a little more.
Event Horizon ~ A tribute to the miracle of fermentation.
Brew what you like. Do this, and you will find your inner brewer.