The more we are, the louder the noise we can make, and the merrier it will be to praise God together. (If wea are not many, how can we make a joyful noise?)
When 10,000 people come together and speak against a policy of the government, it will be very difficult for the government to ignore them. (As a result, your local assembly can influence the culture and policies in that environment).
When we are many, projects become easier and faster to accomplish. Needing less individual effort and personal resource.
“If you cannot go to the mission field, support missionaries with your prayers and personal resources. The more Christians we have around the world, the greater our impact will be.”
How should one chase a thousand, and two put ten thousand to flight, except their Rock had sold them, and the Lord had put them up? -Deuteronomy 32:30
Make it a point of duty to preach the gospel to at least one person every day. 
