How much Malt you need is dependant on a number of things.
1. The Brew Length, i.e how much beer you are brewing.
2. Your efficiency, there are 2 efficiencies really, your Mash efficiency and your brewhouse efficiency.
Your Mash efficiency is how much of the potential sugars you manage to extract from the grains during the Mash
Your BrewHouse efficiency is how much of the beer you brew ends up in the fermenter, some will get lost en route due to dead space etc..
To work out the quantities you need either requires some maths, based on the potential extracts of the grains, or you could cheat and use a Tool to do it for you.
Personally I use Brew Mate,
http://www.brewmate.net/ but there are others. You put the percentages in along with your brew length and dead spaces etcc and it works out the quantities of grain and water required to hit the require SG.