Do eggs make chocolate mousse lighter or more dense? More eggs means?

Yah. Separated, eggs are a magical force. The amount you whip and consequently handle the mousse has the most to do with tenderness, maybe the latter more than the former. If you are aggressively mixing in the egg whites, the mousse will be dense and pudding like. The proper folding technique will allow the most supple, airy mousse possible.

If you whip egg whites and fold them in, they will make it lighter. Using whole eggs will make it richer.