The structure and size of a community is flexible. The number of families advantaged by the personally intensive culture of cohousing does not correspond precisely with the number necessary to fill the employment requirements for achieving maximal autonomy.
A minimum number of families needed to serve an infrastructure able to provide essential food and utilities would be set by the its technical requirements. More would need to be added to the mix to engage and draw income from outside the community.
Once this is achieved the density of families would depend on each person’s comfort level for extended family-type friendships and commitments varies. For some gregarious folks, this size might approach a hundred families. Most folks seem to feel that a community of thirty to forty families is probably right. One solution is to have multiple neighborhoods in a common village. This works rather well in my community, EcoVillage at Ithaca .
The critical point is that the community be large enough to ensure maximal autonomy. And that means indefinitely, self-sufficient in shelter, energy, water, food, access to education, bio-waste recycling, employment and governance.
Creating the engineering, residential and agricultural infrastructure of such communities is technically feasible. Socially, there are decades of cohousing and ecovillage working experience to draw upon.