Human action is defined simply as purposeful behavior. Men act by virtue of their being human. Action can be undertaken only by individual actors. Leisure is a good. Mises derives economic law from this axiom that the study of man is the concept of action.
Utilities are ordinal not cardinal rankings. A supply of a good is available in specific units each perfectly substitutable for every other. The law of (diminishing) marginal utility reflects that each unit that enters into concrete action is evaluated separately. For all human actions as the quantity of the supply (stock) of a good increases, the utility (value) of each additional unit decreases.
An Alice J. Lillie Seminar. This lecture covers pp. 1-78 in the Scholar's Edition of Rothbard's Man, Economy, and State.