Well, I've got 11.1.6 set up in a 12x24 room with three rows of seats. Not only is the rear surrounds a huge improvement, but so was the "rear wides" (aka surround#2) between the side and rear surrounds. That set would not be needed without extra rows, though.
5.1 is ok for one row (although I'd personally still prefer two rears inward from the sides as you can't really get true 90 degree side coverage AND imaging directly behind you at the same time since the side surrounds have to be behind you (even if on the side walls) to image behind you, but then they're no longer directly alongside you... You can remedy that with some front wides on the sides that can then phantom to the sides even without a speaker there, but then you've got another form of 7.1 once again....
Hence, I've got front wides so my sides can be somewhat behind me so true 5.1 tracks can image behind me (between first and second row) yet the arrayed matrix effect puts the flat test tone off to the side of me. The rear wides (between 2nd and 3rd row) do the same for the second row, pulling the side image back to the side of the 2nd row yet allowing smooth panning between sets. It also allows me to put the front right chair closer to the wall (so I can fit three chairs in the relatively narrow 12' wide room as the speaker isn't right up against the chair (and there's an open doorway/drape on the left side so no speaker can go there. This solves the issues completely and everything can image 360 degrees smoothly for all rows (the three center seats anyway; the side seats don't image symmetrically, but they are still inside the overall "sphere").
I would think 7.1 would be needed to get a completely smooth/even sphere for one row, 9.1 for two and 11.1 for three and so on. You might be able to skip rows (say one side surround for every 2-3 rows) with a wider room since the side speakers would create a wider imaging and frequency response window.
That's being picky, of course. Most will be fine with 5.1 for one row.