While not expressed very well by the information, the thumb is considered a digit on a human hand, so therefore the Guardians likely had a thumb and three fingers (thumbs are considered a requirement for using tools, which the Guardians did create and use).
"The Guardians had tremendous dexterity and freedom of movement in their forelimbs and hands" (Language)
Only just noticed this. It might be that they had a pair of opposable thumbs per hand, and two 'standard' fingers. That would add to their dexterity for sure.