@reason
Your game should, IMO, be largely done before launch. This means that, as @bear_necessities says, your theme files are in place and fleshed out, your policies are clear, your chargen guides are ready, your grid is built and desced, and you know what sort of RP you expect players to engage in on day one.
It also means that if you have extra systems - like, say, magic - that said system is LARGELY functional. I opened a game in Beta (technically it still is), and we did sometimes make some changes to the system over time, but the core of it had already been tested, the code worked (...mostly), and the policies surrounding it were things I was fairly certain about.
Personally I would not do that early 'is this even feasible, does combat break all the time' testing on an open game. That's the sort of thing I do with a small, trusted group of friends. Systems that don't matter to gameplay are fine to add later, but if it's important to your theme, I think you should have it at open.
All of these things will change - I update documentation constantly, I adjust policies as things come up, we expand the grid as we see what's missing. The magic system has grown as players have chased different things ICly. Growth is good. That's different from opening an unfinished game, though.
Is there a critical mass of initial staffers? - Yes, but the number depends on your game. Some can run with one person. Some really need a team. You should have an idea of what work you have to do and how much time it will take, and have a team to fit that. If you DON'T have an idea of that work, then you need to figure it out. Understanding the work you're getting yourself into is an important part of running a game.
Critical mass of initial players? - I personally would never work on a game I didn't know at least 6-10 people wanted to play - but I'm sure people HAVE and have gotten traction. Most games that look to be crafted with care and thought these days seem to get a pretty good crowd, so if you feel confident in your ability to design an interesting game to completion, I'd just go for it. But a few early sign ups can def ease the nerves.
An existing metaplot ready for day 1? Metaplot, no. An idea of what characters will be doing and fast ways for them to get hooked into it? Yes. Sometimes this IS metaplot, sometimes it's just flash in the pan stuff, sometimes it's parties. What you don't want is characters wandering the grid going 'what am I supposed to do?' for a week.
I think the process of building a game is in many ways preparation for running one. If you don't have the patience to get all your ducks in a row before opening, I suspect you'll struggle to have the patience to actually do all the minutiae that comes with game running. This doesn't have to be an arduous process - I've helped run games we took from idea to open in 6 weeks. I've also worked on games for 2 years before open. Your ambition, the game's complexity, your available time, your enthusiasm - these all vary. Understanding how they fit together is a really important part of the process, I think.
