My national Advantages are not official in any way.
To use them I reccomend, for the allies. Rolling 3 Dice per power, and letting that player choose 2 of the 3 options rolled on the dice. (For instance, if the US rolls a 1, 3, and 4; it may choose any 2 of those NA’s)
For the Axis I reccomend rolling 3 Dice per power, and letting that player choose 2. And also, to allow each axis power to pick one from its chart that it wishes. So the Axis gets 3 per power, and the allies get 2 per power
2 * 5 Allies (excluding france) = 10 NAs
3 * 3 Axis = 9 NAs
France, in my NAs, has some axis favored, and some allied favored advantages.
For them its really a taste issue. NAs 1 and 6 can not be combined. Perhaps let an axis player roll 2 dice and pick one NA, then the allies roll 2 and pick one NA for France.