The effectiveness of biologicals is simply amazing. For example, miniscule amounts of the legume inoculant rhizobia can influence nodulation, plant growth and development, nitrogen fixation, soil nitrate levels, and plant yield.
If we put rhizobia on a legume seed and plant the seed in the ground, the seed and the rhizobia begin surveilling their surroundings, including each other, ultimately developing the synergistic relationship resulting in all these benefits.
In theory, we could put one rhizobia cell on that seed as a starter inoculum. That cell will divide to create more cells. Bacteria and fungi both do this; in fact, some grow into the plant root and live with that plant all season long. Theoretically, one cell is all it takes; however, due to the harsh seed treatment conditions, environmental stresses, and sampling error, we need to target delivery of many more than that. This is definitely not an argument for second-guessing application rates.
To deliver the optimal number of cells, or microbes, to a seed, we must think about what those cells are experiencing during the seed treatment process: there will be loss on the way. We need to ensure there are enough cells making it through being mixed with other chemicals or salts, being atomized or forced through a spray nozzle, and being stored for a variable length of time.
Product formulations and CFU (colony forming unit) specs are designed to accommodate these factors and still get the number of cells on the seed by the time of planting that will ensure effective starter inoculum amplification, colonization, and performance.
In the end, there is plenty of science to support the efficacy of small amounts of cfus per seed as being effective, owing to the fact that living biologicals reproduce themselves. That said, living biologicals react differently to conventional ag input handling processes than chemistries and inerts do, being much more susceptible to these harsh conditions.
Thus, successful products have been adapted through innovative formulations and delivering far higher CFUs than are actually necessary in order to ensure grower success in the field.