Let's say you're fighting one mob. 20% of the time, he's doing a power attack. This means that 80% of the time, you are free to attack yourself, and don't need to waste stamina holding down block.
Now, assume there are two of them. You are only free to act normally if both mobs are doing normal attacks. In an AND situation like this, probabilities are multiplicative - 80% x 80% = 0.8 x 8.8 = 64% (0.64). Against three, you are only free to act 51% of the time.
SO -
1 Mob - blocking 20% of the time
2 Mobs - blocking 36% of the time
3 Mobs - blocking 49% of the time
This still underestimates the drain on your stamina however, because the number of attacks you block goes up with the number of mobs , too. Not only are you blocking the power attack, but also the normal attacks of the other mobs. So, if we multiply the number of mobs present, by the % of time spent blocking, we get a dimensionless number which nevertheless gives an idea of the relative rate of stamina drain fighting such a pack -
1 Mob = 20
2 Mobs = 72
3 Mobs = 147
Finally, I am under the impression that each hit blocked uses the same amount of stamina, whether it's a light attack from a mudcrab or a telegraphed axe swing of doom from a dungeon boss. If we assume that heavy attacks take twice as long to execute as normal hits, then we're taking even more stamina drain against multiple mobs from all the mobs doing white damage while we're blocking the heavy.
One of the game's big selling points versus normal MMOs is the interactivity of combat. Hit/Miss is not a dice roll. In a normal MMO, it's up to you to position yourself and choose the right skill rotation, but hit/miss, dps and survivability are determined by your stats, your gear choices. This game makes a big deal of how you can actively dodge these attacks, it's one of the first things to appear in the wailing prison tutorial and it comes up on every death recap screen.
However, when encountering packs of mobs, the best approach is not to play that game at all, aoe snare/knockdown/interrupt is the way to do it, or simply burn them down fast with an instant cast aoe before stamina runs out.