by unit size 1+ it means you can take any number over 1.
each unit will use up a rare choice, but they must be of the same type.
example:
you could take 1 stone troll in 1 unit, 100 river trolls in another unit (points are a bit much though

)
or 50 basic trolls in yet another unit, each would only use up a single rare choice each if unit size is 1+