Influence and Multiple Targets
The system is built around a minimum of rolls. When upgrading from a single target of Influence to multiple targets, those affected are all affected by one roll of the dice pool. You do not need to roll for Influence for each target affected.
When resisting the effects of Influence and other powers, there are times when it is appropriate to have an opposed roll of Discipline vs Discipline or a similar pool. This likewise does not need to be a series of rolls for each affected character. Minions can be represented as a Minion group, and Rivals can be represented via choosing a primary target (the person in charge, as we see in the films) and allow the narrative to cover the others falling in line.
You may want to roll separately if there is more than one Nemesis-level character affected, but again it is not necessary.
Magnitude Upgrade Note
It should be noted that the Magnitude upgrade for Influence can be invoked multiple times, so even if the character has it only once, they can still affect multiple targets if they generate enough Force Point resources.
Control Upgrade and Multiple Targets
The Control upgrade allows the Force Sensitive character to add a social roll (Coercion, Negotiation, etc) to the Influence power roll. The force dice and the character's dice pool for the social skill roll are rolled together. This is limited to one target, just like with the base power, unless the character invokes Magnitude to add additional targets as well.