grammarist.com/usage/handful-handfuls/
Handful, handfuls
As a noun referring to (1) a small, undefined number or quantity, or (2) the amount that a hand can hold,
handful is one word (and has one l). Handfuls is the conventional plural, meaning that it is the one listed in
dictionaries and that appears most often in edited writing. Hands full might seem logical, especially in
reference to actual hands, but it tends to give way to handfuls wherever the two words function together
as a noun.
For instance, one would say, “She has took handfuls of candy,” because what’s important here is not that her
hands are full but rather that she took two small, undefined quantities of candy.
Where hands full does work, though, is where the two words don’t work so closely together
and what’s important is the full hands and not the small quantity. For example, one might say,
“He had his hands full taking care of the baby,” because hands and full don’t work together to denote
a small, undefined quantity.