the translation 'summen af mad' is complete nonsense, no one would ever say that in danish.. You can use 'mængde' for amount of food..

The problem is the English, It should be There is not enough food.

My translation "antallet af mad er ikke tilstrækkeligt" is totally different. Is it wrong and if so why?

I agree with Aja. You would simply say: "der er ikke nok mad".