According to the Dragon Quest IX DS case, the game only has one save slot. DQIX producer Ryutaro Ichimuar explains why.
"Because the amount of freedom we give players is huge," says Ichimura, "the data that needs to be saved is enormous."
Thus, it was difficult for Square Enix to create a save file and squeeze in a back-up save file in case the DS's battery dies suddenly. If a back-up data save file wasn't created, players could lose valuable save data. "We're working as hard as we can so data is not lost," says Ichimura.
"In the case of the Nintendo DS," says DQ creator Yuji Horii, "the scariest for save data is when the battery dies."
Instead of another save slot, Square Enix had to use that space for back-up data.