The short answer
One night gives you the essentials: the camel ride out, sunset from a dune, a dark sky, and sunrise. Most of our Sahara trips use one camp night. A second night buys a full day in the dunes to walk, sandboard or simply stop, and we suggest it for Erg Chigaga, where getting there takes longer.
In more detail
The night itself has a set shape. You arrive in the late afternoon, climb a dune for sunset, eat dinner, sit by the fire with music, and sleep. Before sunrise you are woken for the best light of the trip, then ride back. That is a full experience, and on a 3-day trip from Marrakech it is the one that fits.
The second night changes the feel. The day visitors come and go, and you are left with a quiet camp, a long afternoon and the chance to walk out into the sand on your own. It suits travellers who have come a long way for the desert and want more than one sunset.
We rarely suggest more than two nights unless you want a multi-day camel trek away from the camps. If the drive is the problem rather than the nights, one night in Agafay near Marrakech is another answer.