The instructions on the box call for three eggs and that's what the first try used. It seemed just a little bit too cakey and dry to me.
Reducing the amount of egg made it just a bit richer and moister, but not crazy-rich. Of course, I'm using jumbo sized eggs, which may make a difference. If you're using smaller sized eggs, three is probably going to be fine. whip them thoroughly first in the measuring cup before adding them into the mixing bowl with everything else.