Multiple hits come in two types: Number of actions taken, and animations.

Generally, the majority of abilities are single action, but there are several abilities which involve multiple actions.
Ex. The description for barrage states "Physical damage combo 4x to random enemies." In the arena, each action caps at 999 damage, before chain.

Dual wield doubles the amount of actions of any physical ability. Note that variable action abilities such as gamble attack are simply doubled.
Ex. Both weapons will hit the same number of times (as opposed to being rolled separately).

Animation can either be unique or taken from the unit's attack animation, depending on the ability. This will determine the amount of hits the ability will perform.

Weaving a large amount of hits from different units (multiplied by dual wield) is important to creating a high chain value.