Three of them actually. The ones used in gameplay are the small cannons that were seen in the cutscenes with the Covenant destroyer. That why we could use the "MAC Blast" repeatedly ingame and why it doesn't do as much damage as it does in lore (Equivalent of a nuclear blast in atmosphere).
Necromancy again: The Spirit of Fire is confirmed to have three 530 cm modified Magnetic Accelerator Cannons mounted just below the bridge. It was in Halo Mythos, page 70.
Also, ReDquinox, are we forgetting about all the Marathon-class ships? Or Punic-class supercarriers? Or Valiant-class super-heavy cruisers? Or Halberd-class destroyers? Heck, anything heavier than Halcyon-class sported more than one MAC. The Infinity is different, because its guns are described to be stronger than Mark V "Super" MACs as they were the only ones to manage to puncture the Mantle's Approach.