Critical mass is one of those intangible things in worship that are vitally important to the environment that is created in worship. To give you an example of how important critical mass is let’s say you went to a worship service where there were three persons present. It came time to sign the first hymn. Unless you or the other two people were great singers then the hymn would probably not turn out too well. Contrast this to being in a large church or at the Annual Conference when a hymn is sung and the singing almost reverberates in your bones or gives you chills down your spine. In one place there is no critical mass and in another critical mass has been met. It makes a difference.
It is important for the church worship service to have critical mass. This was brought home to me personally when we began a new church and we had 234 people the first Sunday. What a great start, right? The problem was we were in school auditorium that seated 1,000 people and though we roped off sections it seemed like there were 40 people instead of 234 people. In our case, and in churches where there is fixed seating, it is hard to address this issue apart from just hoping more people attend worship. But in churches where the seats are moveable a balance can be achieved. (BTW, we ended up moving from the high school auditorium we were in to an elementary school cafetorium where we controlled the number of seats we put out and it made a difference).
There is no general rule of thumb when it comes to critical mass. You will know it when you see it since much if it depends on the size of the room in the first place. However The general rule I have always heard when it comes to space in general is that when you get to 80% capacity you need to start adding seats or adding an additional worship service. This is true for other areas in the church such as parking and educational space.
If you have the opportunity to control the seating in your space then by all means do so that you can garner the elusive feeling of critical mass and people feel like something good is going on in your church and they are not waiting for you to go to the back and get out the rattle snakes and special kool-aid!
But don’t go too far like I have seen one church do recently. In an effort to garner critical mass they took away chairs when attendance was down and they far exceeded the 80% rule. People couldn’t find seats! People’s personal space was invaded. If this becomes the case for too long then regular attendees will not ever invite anybody because they will feel the church is already too full. So get some critical mass, but not too much!