How much tetanus immunity is a person who has received 1 dtap conferred? seems to me it would be 100% each time for the tetanus portion.

But it isn't: Some vaccines provide nearly complete lifelong protection with one immunization. Unfortunately some do not. They only provide partial protection and must be readministered at regular intervals to maintain protection. And the reasons are complicated why this is...there are textbooks written about the immunology of vaccines. It doesn't change that fact that sometimes, multiple doses are needed.

Body decides: The simple response is the body decides. For most vaccines that are developed against bacteria, the immunity is more often short term and often requires boosters (eg.Tetanus, Diphtheria,Pertussis,Pneumococcus). Some viral vaccines like measles can confer lifelong immunity to some people but may require more than one dost for some.