It's possible to use Helping Hand multiple times in a turn. The easiest way to do so is in a format where triples exists (either gen 5 or 6), where you can simply have both of the edge teammates use it. In 7 forward, you're limited to doubles, but that's also the generation that introduced the move Instruct. Hence, if both of your opponents help out by Instructing your Helping Hand user, you can now get up to 3 Helping Hands to power an attack, instead of the 2 from basic triples. Oddly, if you Instruct the same Pokemon to use Helping Hand multiple times in the same turn, the "user is ready to help target!" message appears twice in a row on the second use, and three times in a row if there's a third.
We have been able to verify damage rolls that show each Helping Hand is considered separately: two of them are worth a 2.25x damage bonus, and three are worth 3.375x. In the 3-HH case, even though the "ready to help" message appears a total of six times in the turn, only three of them indeed count: if it counted all six, that would be more than 11x damage.
