I'm not sure the number matters as much as the "who" the players are. When I started playing I was in a conference with, I believe, 7 people...5 of them were mushrooms who barely interacted or did/said anything. It was the other two who were responsible for helping get me hooked, get me thinking differently about recruiting and positioning and such, etc.
Definitely agree that being a newbie in a stacked conference of 10 others can make it extremely hard to gain traction and improve (although not as much in 3.0 as it used to be...). I'm not sure what I'd say the "top number" would be...maybe 7-8...that leaves enough Sims that you're not having a donut hung on you every season in league play. But I'm not sure I'd look at the the number as much as I would the vocalness of the chat board. The leagues where folks are talking actively are the ones where people can get help and learn things the fastest, are the more social groups that add a different element of fun, etc. Being in a league with atblack was always awesome for that very reason -- he made it fun to have your *** kicked by him 1-3x/year by all the other banter.