I was reading an article from HT talking about growing in the small greenhouses. The article said that you could expect to fit as many as 6 plants of reasonable size in a little 12 by 12 greenhouse. How many of the smaller auto plants could you expect to fit into the same space?
I grow 4 autos in a 3x3 tent. A little crowded sometimes, but not to bad… If it were me, I would start with 12, and see how things go, by the end of the first batch, you should know how many more you can fit, while still leaving enough room for misc equipment and room to walk.
Ultimately if you gave each one a 4sq ft space (I found most mine do not take up much more than that), you could do up to 36 (if my math is right, but I am kind of toasted right now). It would be crowded and a pain to water as you left no walking isle, but could probably be done…