From Wikipedia:
Variation in tone can be achieved by striking different distances from the bridge and using different parts of the thumb (especially fleshy vs. bony parts).
Variation in chord texture can be achieved by selecting different strings to strike.
Seems like if you're asking if it is correct to either hit all of the strings or only one, the answer is yes, it's fine. I imagine it would be more difficult to hit all the strings at once, but that's where you experiment with other parts of your hand instead of your thumb.