Two would definitely be preferable, if we have the numbers. One staying near the center of the campsite, and one moving- they can trade off though the course of the watch to prevent boredom and to keep the routine a bit varied for any intruders. As for having a campfire, I think we might want a small one- but have a lantern or sunrod ready if there is trouble, since not everybody has darkvision.
For what it is worth, Ipshivi doesn't need food either, so she will actually take her two hour rest essentially immediately once the party stops moving for the night, while everyone else is going through the drudgery of setting up the camp and having dinner/evening decompression time. Figure she'll miss maybe half of the first watch, then stand guard for the rest of 1st and all of 2nd and 3rd. Once she starts watch, she'll find a good hiding spot (most likely perched in a convenient tree)- she'll choose her spot carefully (taking 10 on her Hide check, for a 25 +Invisibility)