I've run a 20-team league and we're in our fifth year. I'd say we have a core of about 13-16 owners that are reliable to come back and about 10-14 that are truly active from start to finish.
The problem is, once you're out of contention (top 3, top 5, maybe top 10), it's hard to put in as much time, especially if you have other leagues where you are more competitive. If you're sitting in 15-30th place, are you really going to stay active.
It'll take a couple of years of building up the league and bringing in a 1-2 new "keeper" owners, but the product has to be worth it as well.
My league is a 28 category league and since there are some stats that contradict each other, you just can't approach your strategy the same way you would most other leagues and a lot of owners that do well in other league don't in this one and they find that a challenge the first few seasons. That's a selling point. We also have a chat box with some members putting in a of funny/inflammatory/thoughtful information (drama always sells and seeing an active league does too). Most importantly, we have a three-commish system with clearly stated rules and procedures to make sure that everyone understands fairness is number one.
Still, with all of that (and some other stuff we do during the season), it will take a couple of years to really build up a solid league...and that's with 20.
I just think #15-#30 will check out early and you'll end up with maybe 10-13 active players out of 30, which can be discouraging itself. No one wants to put already limited time into a league when they're trying to jump from #24 to #18.
Start with 20 and see from there. I've got other tips and ideas if you're interested.