"The workers are in the harvest" is a slogan we often use. While it's often very true, we ought to give a caveat. We can be led to think the first movements started will generate all (or at least most) of the workers needed for the remaining task. But this is not necessarily the case.
I've found the reality to be a little harder. Most of the time, movements generate enough workers for their own harvest--"the workers for this harvest are in the harvest." Sometimes, more rarely, they generate a little more besides for nearby harvests, in a "local flooding" effect.
When we look at the "whole world," the waiting harvest is big, huge, vast: thousands of millions. I am tracking 16.1 million believers in 131 active movements, for an average of 123,581 believers per movement. Historically, I've seen most movements tend to max out at about the 100,000 level. If this is a "general" rule of thumb, then ‘at least’ 20,000 movements needed.
I say "at least" because we will actually need more than that - perhaps double or triple that number, with attrition, fizzles, and movements that never get very big.
To start those movements, we need a similarly vast number of movement workers. But the “kind” of workers necessary to cross-culturally start a movement--workers who can operate at the "E-3" level--are of a different kind from those needed to work in a same-culture or near-culture movement (E-1 to E-2 level). The Status of Global Mission 2017 estimates (line 48) the number of foreign workers at 410,000, or 1:5,690 believers worldwide (and few of these are trained to start movements, or even thinking about it). By contrast, national workers are estimated at 12.2 million, or about 1:200 people. They are far more common, it seems. While the data is thin and not conclusive, these two ratios suggest it will be easier to find workers sufficient for the current harvest, than to find the "apostolic" types that can cross languages and cultures.
That doesn't mean it's impossible. It just means it won't be easy. We have to continue to be intentional about finding and raising up cross-cultural apostolic workers and sending them out. These workers may come from within the harvest, or they may come from other parts of the world.