Can you please tell me where can I find this in the rules ?page 4, basic game concepts - using units
5. A Unit that is Ready and not Wounded may be activated for one of its abilities.
a. To activate a Unit, put its Command token on it – the Unit is no longer Ready, it is spent until the end of the Round.
b. Then choose one of the abilities displayed on the Unit card and apply its effect. You may combine it with the effects of other Deed cards, Skills and Units.
The bold sentence would make no sense if only one unit could be used per turn.