In Richland County SC there are 12,681 reported Roman Catholics and 7 RC houses of worship (HOW). That averages out to 1,812 peole per HOW.
Source: City Data
So what you may ask.
The density of Roman Catholics per HOW is far greater than any other Christian denomination. I don't see a lot of RC mega-HOWs in Richland County, so I suspect that in Richland County they are overreporting.
There are multiple Masses at each HOW so all can be accommodated.
Catholic churches often have more than one mass.
babies and the elderly often cannot attend. Since it is South Carolina, some may have to travel a distance to go to a Catholic mass, or miss mass because of work, so may not always be able to attend.
Statistically about 30 percent attend every week.
30 percent of 12000 is 3600 Catholics, but if you include those of us old or young or sick, or those who go at least once a month it is probably closer to half of those 12000 who are pious believers.
That averages 600 people per church per Sabbath...and since most churches have at least two masses, that means 300 or less per service.
sounds about right.