I love a good strategic project management question!
With the scale of your project, here are a few key things to keep in mind:
1) Your mixing station should be changing locations as the project progresses, and therefore, easy to move. Having a nearby station to your building crew will reduce fatigue when the material is in transit, thus, increasing efficiency and morale.
2) Your source of earth (and rock/sand if needed), should be as close to the mixers as humanly possible.
3) Bag crews (of single dome structures) are usually able bodied groups of at least 4 people per dome: 1 mixer (2 mixers on ground levels), 3 bag layers (2 layers on ground levels). Keep in mind that each dome layer will also need to be tamped, barbed, and checked with the compass as well...
4) Inexperienced or slower building crews will take approximately 4-5 weeks per dome to build, while experienced crews should be able to build this in 2.5-3 weeks. To achieve your goal (assuming inexperienced volunteers over 18 months), you would need to build about 4 domes per month; thus, you'd want at least 16 full time, non-stop dome builders.
5) Crews will inevitably be on different phases of building. Some crews will need bag fill, and some will need plaster or slab mix. Some crews may need 2 mixes before another finishes 1 mix. For this reason, multiple mixers per station is vital for building efficiency and mix integrity.
6) A 6 cu.ft. wheelbarrow can hold (1) 6 cu. ft. mix (assume 70-75% @ full capacity) comfortably, but can be quite cumbersome for many to move. Increasing the size of your mixer assumes that teams will ideally be moving in near synchronicity as to not let the mix settle for too long.
For this project I would personally recommend, that there be 2 different mixing stations with (2) 12cu.ft. mixers apiece (with one extra on standby). Each mixing station would need 3-4 people to be effective; once plaster mix comes into play, the additional task of screening the material will propagate itself, and the extra (wo)man will be helpful. An experienced mixing team should be able to communicate and execute the needs from each crew and deliver mixed material as well.