A series of posts on the art and craft of fannish volunteer-run conventions. These are my opinions and my experiences. As always, YMMV. All of these posts are available here: http://www.boston-baden.com/smofs/conrunning/
A few caveats about what I'm about to say.
This is my own opinion, about the sort of conventions I like to be involved with. They tend to have more features as they get larger. I think this applies up to about the 5000-person size of conventions, as above that you can benefit from some economies of scale.
Make a list of everyone who's making the convention happen. The directors or chair(s), the division and department heads, the assistants and seconds, the staff and the volunteers, the artists who contribute to the convention. (I like to have that list anyway, because I want to thank each of them, but that's not important right now.) Count the names.
Now consider the total number of attendees. All the unique badges issued. If one person gests a badge for being staff and a different badge for being on program, that only counts as one person, not two. Divide the first number by the second, and express it as a percentage.
I think these conventions work best when that ratio is around 10%. Eight percent to ten percent, anyway. World Science Fiction Conventions tend to have 10% or more — a brigade of volunteers, many of whom have staffed Worldcons before, and when they get to town they get moving and things happen. For most of the years I was chair of Animé Los Angeles, we were running close to 9%.
If your ratio is closer to six percent the convention can still function but you're going to start seeing a lot of burn and churn. If you're down around four percent, your convention is on life-support and fixing your recruiting and all of the other systemic problems are your top priority. Case in point, by Anime Conji's second year they were at 6% and falling and the cracks and fault lines were clearly visible.
I'd like to emphasize that I think the ratio can drop off when you get in the neighborhood of ten thousand or so. You should still be mindful of the overall numbers, though. And if your staff and volunteer numbers (possibly including hired outside contractors such as tech or security) are declining, even as your overall attendance is rising, I would view that with alarm.
Now growth patterns are wildly different in the anime convention world vs. science fiction fandom. Most anime conventions have a disconcertingly steep growth pattern to them. If your first three years you hit 600, 1300, and 1800, then you have to put a lot of care and attention into recruiting to support the convention's increase. You also have a significant task in socializing the newcomers, educating the barbarians at the gates as to the convention's norms and etiquettes. It's a wild ride.
You should always be on the lookout for good staff people.