I have at least 20 cacti in 4" pots and 6" pots under a 100 watt grow light. It BARELY lights them all. Ive got 15 succulents under 2 40 watt grow lights and a few that fit nowhere that have to sit along the window sill. Every day I rotate and move plants around so everybody gets time under a light. But...they are all growing very nicely under artificial light! They all do get 2 or 3 hours of real sunshine every morning. And I just brought my 2 Barrel cacti inside, so no more plants even though I want more!!Mammillaria are super easy to fall in love with! They're just so cool, but easy to care for!
I'm sure a few more wouldn't hurt...