What happens if I have less than three spaces on my bench when using Politoed's "Roll Call" attack? Or what if I don't have any room on my bench at all?
You can use the attack no matter how many Pokemon are on your Bench, but you can only get as many Pokemon as you have spaces on your Bench.
Source: Guardians Rising FAQ (2017-04-22), TPCi Rules Team (2017-05-04)

Relevant cards

Note: All applicable cards may not be listed below.
SM2_25