Can you clarify Maria Hill specification: Is the max number of copies for a SHIELD support is mandatory, or…

You can think of Maria Hill’s deckbuilding rule as “all or nothing.” If you choose to include off-aspect S.H.I.E.L.D. support cards in her deck, you must include the maximum allowed copies of 3 different cards by title; otherwise, you would include no off-aspect S.H.I.E.L.D. support cards. (For example, in her preconstructed deck she has 3 unique off-aspect S.H.I.E.L.D. supports, each one of a different aspect.)