How many eggs can you carry in both hands without a tray and without dropping any. I've tried it and it's about 12 (depending on the uniformity and size of the eggs). Most agricultural units of measure have a similar basis depending on what a peasant could carry.
In theory, you could carry 14 eggs. 9 on the bottom row arranged in 3x3. 4 more in the 2nd row arranged 2x2. 1 on top. However, with the invention of extra-huge-jumbo monster eggs, this is difficult especially because the top egg is easily toppled. 12 is more reasonable approximation assuming a disorderly arrangement of the bottom row. (This is what one learns on a chicken ranch).
Note: In some parts of the world, egg stacking is an art form: