From: Leeuwarden, Netherlands
if you look at a unit it will tell you the the percentage of hq staff and HQ power. the power is raised and lowered by how close the HQ is to the unit , I think anything over 4 hex you get no bonus.
82 is right, except that you will get a bonus over more than three hexes. Up to three hexes it's 100% bonus, four hexes is 80%, five hexes is 60%, six hexes is 40% and so on till zero procent bonus. But you can check this yourself on the HQ efficiency bonus on the unit counter (100%, 80% and so on).
Hq's are expensive to make so they're scarce in the beginning, but I tend to make one for every say 10 hexes front line, sometimes less, sometimes more, depends on how dangerous the enemy is there.
So just two HQ's for a loooong frontline is very very low, that'd be easily 10 or so. One HQ with say 6-10 subordinate units is, IMHO, a better strategy. More flexible. And a HQ can have up to 100 Staff or more (depending on how many units they have subordinate), so 10 or 20 seems a bit on the low side too.
Besides, if they are cut off, but their HQ is still within the pocket, the still draw supply from their HQ. That HQ will rapidly dun dry ofcourse, getting no more supply from the Supreme HQ (the original HQ you started the game with).