Will a multitype Pokemon such as Golurk with the "Double Type" Ability count for two different types of Pokemon for Xerneas' "Rainbow Force" attack?
Yes, Rainbow Force checks for the number of types of Pokemon on your Bench, not how many Pokemon there are.
Source: TPCi Rules Team (2016-01-07)

Relevant cards

Note: All applicable cards may not be listed below.
XY8_107