Apart from armor, there’s two defenses your character can have: dodging, and blocking.
Blocking is granted by martial arts and some equipment, it lets you negate either partially or completely the damage of an attack, it isn’t affected by your encumbrance (which means it’s useful even when carrying two duffel bags)
By default, you only have one block per turn, with your best blocking item, or if you don’t have any and you have an MA that grants it, with the least damaged of your viable limbs. This means that while it’s good against one enemy, it’s usefulness quickly dries up against multiple foes.
There are martial arts, however, which grant you additional blocks, that is, you can block more than once per turn: Karate, Tai Chi, Capoeira, Biojutsu, and Niten.
Dodging, on the other hand, is heavily affected by encumbrance, and you can dodge more than once per turn. However, with each additional dodge, your dodging ability is halved until the end of the turn.
Some martial arts give you ‘free’ dodges before you start getting penalties: Karate, Boxing, Ninjutsu, Zui Quian, and Silat. Zui Quian is notable in the fact that it grants 100 ‘free’ dodges, effectively letting you dodge as many attacks as you want per turn without facing diminishing returns.
What this all means is that if you know how many extra dodges and blocks your MA and equipment grants, then you know how many enemies you can safely handle simultaneously before meeting drastically diminishing returns in your tanking ability.
The other side of the coin is, of course, simply being able to kill them faster than they can swarm you.