Can a Hexing Doll be stuffed with the hair of more than one creature and gain the bonus to DC for each?
"fill (a receptacle or space) tightly with something."
You could stuff the doll with the hair of several different creatures and it would still satisfy the reading of the ability. This is definitely something you should ask your DM about. For myself, I'd allow you to do so, up to a reasonable limit.
I'd also say only one, because
1) The language used is singular: "Alternatively, if the doll is stuffed with the hair of a creature, any witch holding the doll increases the DC of her hexes by +2 when using the hexes against that creature."
2) If it were meant to allow multiple creatures' hair to be used, it should probably have specified the "reasonable limit" for number of creatures whose hair could be in the doll.
3) Thematically, dolls or figures used to perform magic on a target generally represent a specific singular individual.