How much water do I need to mix a 80 pound bag?

You say cement and maybe mean Quickcrete or similar ready to use mixture.
Regardless of which you are using, you need enough water (and only enough) to make the concrete workable for use in whatever you are building. I don't know what the target amount is (maybe the bag makes a recommendation) but it needs to be thoroughly mixed with a hoe or similar tool until it has a uniform consistency with no dry ingredients showing. The strength of the mix is affected very much by the amount of water so the less water you put in the better product you will get. Add small amounts of water at a time and you will be fine.

This what the Quikrete website says to add to a 80 lb bag:

For each 80 lb (36.3 kg) bag of QUIKRETE
Concrete Mix to be mixed, add approximately
6 pt (2.8 L) of fresh water to the mixer