As to the poll: 4-6 people is what I shoot for with D&D. I find that is the range that you have the most diversity a single GM can cater to without loosing people in the mix. Any more than that and the players begin to pull off into cliques around the table, which will inevitably mean one or more people getting the short end of the gaming stick.
As an aside: I find that the number of people I feel comfortable DMing for is directly linked to how well I knew the rules and how comfortable I felt with the system; any system. When I started D&D with 1st edition, I liked no more than 3 people. I simply couldn't keep up with more incoming requests than that, and I had not really gotten a hold of how the "job" of DMing was done. As I became more knowledgeable and comfortable, my prime number expanded.
To wit, in those 1st edition days I tried running for 8+ people once and got demolished. Nothing made sense, a few of the players ended up doing things blatantly against the rules at the time and were very selfish. Half of the people hardly got to do anything because I was only able to respond to the loudest or most annoying voice at the table. Now-a-days I have run for up to 10 people and keep it going well. It just takes too much brain power from me, and frankly it's more work than I care to do anymore.
That being said, some systems are simply better with a specific number of people and some people are more comfortable with X number of people in a room, no matter whether you're playing D&D or having a party. For instance, I find 6-8 being great for super hero games.