Major castes are produced only when the colony reaches reasonable size. It depends of species, I don't know which one your are talking about, but I doubt any ants would produce soldiers in beginner colony of 12 workers. I think you need to wait until colony reaches size of 100 or more...
The colony needs in deed a reasonable size. My Pheidole pallidula had there first soldiers(majors) when they had about 50 workers.
That is the first thing a colony needs, enough workers ;) but the second was really the question, what determines a major and a minor. ;)