We'd like every disciple to make disciples. The result, in the words of David Platt, is multiplying churches--every time. The reality is, not every disciple will.
And the fact is, not every disciple must make disciples in order for the church to multiply.
If disciples who make disciples only double (e.g. each disciple makes 1 disciple) - then we have a problem, especially if only a small number do. This is akin to the challenge a nation faces when a father & a mother only have one child: it's below "replacement value."
On the other hand, if disciples-who-make-disciples make groups, then even if only a small number do, we'll see growth.
Assume out of a group of 100 believers, just 5% start new groups of disciples - but each group has 5 people. 5% of 100 is 5 disciple-makers; if each has a group of 5 new believers, that's 25 new believers. The church just went from 100 to 125.
If this is duplicated the next year, 5% of 125 means 6 disciple makers, 30 new believers, and we've grown to 155.
By year 10, at 5% disciple makers each year, the church went from 100 to 875. By year 20, it's at 10,000.
I'll grant you this isn't huge, but if you did this in a population segment of 100,000, it means you reached 10% of the population in 20 years. What about smaller numbers?
2% planting groups of 5 gets you to 565 by year 20. 3% planting groups of 5 gets you to 1,420 by year 20. 4% planting groups of 5 gets you to 3,425 by year 20 (3% of 100,000--I know some places that would be thrilled with that).
What if 5% each planted groups of 10? By year 20, you'd be at 316,000 - you would have easily spilled "over" the 100,000 population segment and swamped the adjoining districts.
Don't think you have to have everyone. Yet small improvements - from the % of disciple makers or the group size - can have incredible results over 10 or 20 years (a career). We'd like to have everyone doing it, but don't think you can't make a big difference with a Gideon-sized group.