Does anyone have info why historically nations didn't put as many HMGs on their planes though? I think I read that the British did so for a while, and it helped for hitting the enemy at all by just throwing as much stuff at them as possible.
I'll share the little I know.
Answering your first question, this is related to how each nation approached MGs in the first place. The Germans, for example, were trying to make the 8mm Mauser the standard, to simplify logistics. So, all their rifles, MGs, and following it the armament of aircrafts, were made in 8mm.
At the same time that the first BF-109s were leaving the factory, the Germans were research ways to fit the 20mm cannon in it. Because they knew that a pair of 8mm MGs wouldn't be enough to shot down enemy aircrafts in a fighter.
The USA approached this, by making two separated MGs. One heavy, intended to be mounted in vehicles (the .50 cal) and a light one to be used by the infantry.
The British started in WW2 using .30 cal in their aircraft, if I'm correct. But later they switched to .50 cal and introduced the 20mm cannons, because it was evident that the .30 cal was too small to inflict damage, fast enough, to shot down an enemy aircraft.
Were the ammunition you need to carry to heavy, was there to much wind resistance when mounting so many HMGs, were the HMGs themselves so expensive/needed elsewhere?
The ammunitions and the HMG were indeed heavy and would impact the aircraft, but in its ability to climb, range and some times their turn too. Because most aircraft's weapons were mounted internally in the fuselage. The only case that it would impact negatively the speed of the aircraft (drag), would be gun pods. One good example would be the gun pods installed in the
MC. 202 EC, or the BF-110.
The HMGs weren't expensive, per say (all military equip. is expensive IMO), but many aircrafts weren't designed to fit them in the first place, so in many case you'd find these transitional solutions, until de engineers were able to fit the weapons inside the fuselage.