The 20-mission limit to me just seems like a standpoint to start rule from when confronted with a proposed campaign. Depending on the scope, if it was a really ambitious campaign with not enough work being done then there would be no exception to the 20 mission rule and that project would have to show considerable effort to be hosted then. However if for example you already have 10 decent missions finished quickly and you plan to go beyond the 20 limit, I could see how that could be an exception. As Goober said, this is a hypothetical statement, but since people here are looking for that for some reason, I'll offer it.
Compare this the one-month limit monkey rule. It can be elongated if the offense is more severe, and possibly (depending on whether the Admins will forgive that much) shortened if the offense is very minimal. However, Goober may have different logic behind this, but common sense states that this limit has some degree of reasoning involved depending on the situation. To my understanding, this reasoning will be held the same way for the mission limit.
The 20 mission rule seems good to me, as it just seems like a general standpoint to make rules depending on the situation. Aim your projects to not go over that limit and you'll be fine. Exceptions, if ever made (as I could be wrong with Goober's logic behind the rule), should only be rarely and if an ambitious project is showing considerable effort.
That being said, get out of here, go FRED a campaign, and worry about the limit when the time comes. Just keep in mind that exceptions will rarely be made (again I'm speaking on behalf of Goober, so I could be wrong.), so just aim not to go over 20. It's quite simple, really.