if someone plays a fuse and someone plays a counterspell are both spells countered or 1 ?
Both will be countered, since you're casting it as a single spell.
They are both countered. When you cast both sides of a fuse card it is considered as one spell, with characteristics of both halves of the card. It still goes onto the stack as a single object, therefore a counter spell would counter the entire thing.
I couldn't find a ruling but if you look up the rules of the dragon's maze mechanics it is explained there.
Any time you could cast a split card with fuse, you can choose one half and cast it, just like any other split card. And if you're casting it from your hand (as you usually are), you have the additional option to cast both halves as a single spell by paying their combined cost. Instructions on the left half are processed first, then the instructions on the right, and targets for the two halves are chosen separately.
If you were to give it flashback, could you cast both halves from the graveyard?
Nope. Fuse's text states it has to be from the hand.