This is a good question. I would say that there are two protagonists in the short story. First, you have Della. The story opens with a depiction of her poverty, as she does not have enough money to buy her husband Jim a present for Christmas. O. Henry states:
"One dollar and eighty-seven cents. That was all. And sixty cents of it was in pennies. Pennies saved one and two at a time by bulldozing the grocer and the vegetable man and the butcher until one's cheeks burned with the silent imputation of parsimony that such close dealing implied. Three times Della counted it. One dollar and eighty- seven cents. And the next day would be Christmas."
As we read further, we see that her husband Jim is in the same boat. He does not have enough money to buy his wife, Della something nice for Christmas. As the work progresses they both sell what is most precious to them to buy something for their spouses. In light of this plot, there are two protagonist - both Della and Jim.