Ahh... this is a topic we have experience with...

Our current project has 22 characters in the works currently, and its not even started public recruitment.

Beyond what [@Jig] said, you also need to be able to manage time. With a large player count, you will end up with players who are hours (or even days) ahead of the others. If you don't somehow keep track of time, people will end up tying knots on the timeline. 

Also, if you've got a larger group, co-GMs are invaluable. Nobody is available all the time, all days. Sometimes RL comes in the way. Sometimes you're just tired. Sometimes you need to discuss plans with someone without having it public knowledge among the players. That is where Co-GMs come in. They can step in when you can't. They can see things from a different angle.

Co-GMs usually have strengths you do not have. Some might be better organized, others are better at dealing with people. Some might be masters are comprehending and utilizing lore, or perhaps good at laying out plots.

In our opinion and experience (there are several schools on it) a co-GM should not be kept in the dark. Its better for the secret plan of the RP to be known to all the GMs than to be solely the property of one. A GM that doesn't share plot with his (or her) co-GM(s) doesn't really want co-GMs. (s)he wants minions. Not atypically to take care of the boring, tedious parts. Not exactly fair to the individual chosen. Such burdens should be shared. That gives better dynamic and flow for the story.

Clear, easy-to-read sheets are also necessary, and don't be afraid to absorb ideas from your players on that. We've learned plenty from our players over the years.

for recruiting new players, longer RPs should also have summaries.

For the inevitable dropouts, you need a system for exiting player characters, and if death is a tool you plan to use, don't just kill all dropouts... it ruins the effect when you need it for the story.