if using concrete, ratio is 1 part cement to 6 parts sand.

It depends on how big your mold is. I suggest you lay the mold on a flat, dry surface and fill it with the dry concrete mix*. I'd add a little extra to be on the safe side. That will be about the amount you need: gather it up, put it in the bucket or whatever you're going to mix it in, and then, when you're mixing it, add only just enough water to give you a good consistency - a mix stiff enough that you can't simply pour it into the mold - it's better when you have to pat it in. Err on the side of minimum water - too much water makes concrete weak.

*I assume that you'll be buying a bag not of cement, but of concrete mix. Concrete mix is cement already mixed with sand or other aggregate, in the proportions mentioned by Potty.