Luke 9:14 For they were about five thousand men. And he said to his disciples, Make them sit down by fifties in a company.
15 And they did so, and made them all sit down.
16 Then he took the five loaves and the two fishes, and looking up to heaven, he blessed them, and brake, and gave to the disciples to set before the multitude.
17 And they did eat, and were all filled: and there was taken up of fragments that remained to them twelve baskets.
50 people is an ideal number for a small church congregation. You have enough people to have children's work, young people, couples, etc. But not so big that everyone won't get to know everyone else and you will have a very nice little church life. You are small enough that you can actually meet in someone's home, or a conference room at a hotel. The church is small enough that everyone will feel they need to participate in the service and the work. People will invite each other to their homes for fellowship. I have met with churches that had 200 active members, another with 2,000 members, and one with 50 members. The church with 50 members grew to 100 in about a years time. The church with 200 did not grow very quickly, but then 100 members went to another city to raise up a church and in about 18 months they had grown from 100 to 200 members again. The church with 2,000 didn't grow at all. It seemed for everyone who came into the church someone else left.
It seems to me, from my experience that you want the church meeting to be a mentoring. You start at 50, when you get to 200 it is time for 100 to go start another church. That means while you are growing you have to train the members in the service and work of the church. People sharing testimonies, people sharing the messages, people taking charge with the service to keep everything operational. By the time you have 200 you should have a meeting hall. The church that started with 50 members, we got a meeting hall when we got to a 100 members. If 50 people come from a larger congregation and establish a church and a year later they have 100 members the original congregation should be able to assist them with getting a meeting hall. Also you should do all the construction yourself with your own members, that cuts the cost by 65%. You will also find that construction is what some of the members can do as a service. Hiring workers robs them of their service and letting members do the work themselves makes them invested in the building and realize this is their church.
Now imagine these 5,000 members are now ten congregations and in one year they double to 10,000 members and shortly after that the ten congregations become 30 congregations. No wonder people are wondering who this is.
18 And it came to pass, as he was alone praying, his disciples were with him: and he asked them, saying, Whom say the people that I am?
19 They answering said, John the Baptist; but some say, Elias; and others say, that one of the old prophets is risen again.
20 He said unto them, But whom say ye that I am? Peter answering said, The Christ of God.