Metcalfe's law of network valuation makes absolutely no sense if you are talking about a small community being able to absorb new people.
You could say it just belongs in the realm of computer networks, but I think if a thing is going to work, well it ought to track reality in some way.
So, obviously, 20 people can't take on 180 people and have the original 20 successfully turn the total 200 into something similar to what the 20 had.
But, as per Odlyzko, 20 people could take on 6 people and integrate them into their society, and subsequently the 26 could manage around 10.
So, if you have heard me suggest that God could be telling you to do X, and the devil, knowing your predisposition could show up and say evangelize, well, now you know why I am saying that. You can evangelize yourself out of existence.
It is important to understand what is on offer in America. The effect of women entering the workforce has been pretty devastating on wages. Additionally, the nature of the workplace has changed. Men are in danger in the new environment, and H.R. seems hell bent on creating interview processes self-respecting men can't even go through, much less complete. The proliferation of credentials (degrees or otherwise) function as a bar between whatever work a man can actually do and the job. Meanwhile, the legal nature of marriage in this country means a man has no rights, but rather he gives a woman license to destroy him through the courts if he gets married. If you are concerned about raising your children as you see fit, well, everything is precarious.
So the evangelist may meet with temporary results, but the realities keep coming back. The recidivism rate of people who have been evangelized is rather high because the incentives don't change and Christian communities have not been built up enough for different people to have a real path within them.
The math above, I think, gives a more realistic view of what could be done. All of the issues with it suggest a sober, measured attempt at growth. The nature of the 20 must be determined in the first place. When does 1 of the 6 become one of the original group? How are these things determined? What's the time frame?
Interesting things happen at 150, which also happens to be Dunbar's number, and may be the appropriate size to shoot for first, before any outright evangelization. The valuation suggests 150 people can support a little more than another 150. 105log(150)=326.413688858. 326-150=176. I can imagine a communal form of mitosis here, where the kind of fast growth evangelists like to dream of actually might be able to happen.
It just struck me that this is like a weird form of the parable about the seeds. American evangelists throw seeds on barren ground. If a plant comes up, it must fend for itself.