I'm with you, the 3 admirals and the vice admirals were enough imo. But don't forget, Whitebeard and his crew had his own shichi too: Luffy, Jimbei, Ivan, Boa, Croco... so that leaves us with Whitebeard, Marco, Jozu, Vista, Ace (don't forget the BB pirates) vs the 3 admirals, Garp and Sengoku.
But yeah, the marines had the numbers and position.