This is a case where the context can make a difference. In simple terms, "much more", is a lán níos mó, or i bhfad níos mó (or a number of other, similar constructions).
When you say "You need much more", you mean a large amount (a lán níos mó), but when you ask "Do you need much more?" you might be asking about a small amount - "Is that nearly enough, or do you need just a little bit more?"
So when you ask "do you want much more?", if you mean "Do you want me to give you a large amount more than you already have?", then an bhfuil a lán níos mó uait? works but if you really mean "do you have enough?" then an bhfuil do dhóthain agat? is a better translation.