What is Influence and what is it used for?
Influence is granted by the apartments in your cities and determines the number of troops you can train.
To see your current Influence, click on one of your apartments in your Neighborhood view. In the apartment screen, there is a tab located at the top labeled 'Influence'.
In the Influence section you can see your total Influence and how that total is achieved through the apartments in your cities. Higher level apartments will give you more Influence and allow you to train more troops at once.
In your Hideout, you can see the required amount of Influence required to train a unit.