How many members were there in the church at Corinth? The churches of Rome or Galatia? Any one of the seven churches of the Revelation? We don't know.
Why don't we know? The Bible doesn't tell us.
Now, the Bible does tell us "...all things that pertain to life and godliness" (2 Peter 1:3). So, the more important question is: Why would we ask?
"Satan moved David to number the people." God had said not to number the people. He didn't want Israel to trust in himself and his own strength.
Gideon learned that his 30,000 mustered troops were too strong. God whittled it down to 300. That way they could face an army of over 100,000 and know for sure that God had delivered them rather than their own military might.
The disciples asked Jesus whether it was few who would be saved. He replied, "Strive to enter through the narrow door. For many, I tell you, will seek to enter and will not be able." He also taught, "The gate is narrow and the way is hard that leads to life, and those who find it are few."
How many are "a few?" Peter said "a few" were saved in the days of Noah. That means eight. And Noah had preached the gospel for a hundred years by that time.
Jonathan son of Saul understood that "...nothing can hinder the Lord from saving by many or by few," as he and his shield-bearer overthrew a Philistine garrison.