Gardner420, I just harvested about 25 autoflower plants under a 1000W MH/HPS switchable in a 4 foot by 4 foot by 7 foot tent if you want to use that as a reference. The grow went swimmingly well despite advice to not grow this many in that space. It wasn't all daisies though; it was a LOT of work, I ran into a fair share of troubles with plants and I wasn't able to give them the kind of attention I would have liked had I gone with fewer plants. The question that you should be asking isn't how many plants you can fit under the lights, but what STRAINS can you fit under the light. I grew some White Label Skunk Autos that grew like 2 1/2 to 3 feet tall and threw out TONS of stems and branches. Any shorter plants within say a foot would have been shaded out or had bud on bud action. I grew some 60 Day Wonder that grow less than a foot tall and grew basically as one gigantic bud from the growing medium up to the cola, with very little branching. I grew some Dinafem White Widows that grew such leafy arms anything next to them was gonna get a face full of leaf. Depending on the strain, you could fit a whole BUNCH, or not many at all.
Also depends on the size of your growing pot and the shape of it. I could stack 4 (maaaaybe 5) 2 1/2 gallon pots next to each other in my tent; or put 17 gallon tubs in there and let multiple plants fight it out (which worked alright, I would take a few extra steps into consideration on my next bout.)
I'm a noob grower, I just finished my 3rd grow and I have grown about 34 plants total so far. It's just my personal opinion based on my experiences, but stick with fewer plants and really learn how to grow them well. Learn those pH balancing techniques. Learn the symptoms of plants. I have several sites bookmarked with full color pictures of deficiencies and bugs and diseases. Out of 25 autoflowers on my 3rd grow, I have 2 plants total that look like something you could see in a magazine. I really think had I gone with fewer plants and taken more care of them, I could have maybe not necessarily better results, but more consistent results.
Just food for thought and my

Good luck and best of luck to you

It's amazing when you get to reap the rewards!