Halo: Ghosts of Onyx
Page: 139
Paragraph: 3
"He[Kurt] wished whatever was happening had started last week- when there had been three hundred spartans on Onyx. the tactical situation would be much different".
not all of the SPARTAN III's were killed, just the first Class, then Kurt improved and enhanced their skills later. the battle at Pagasi Delta started on July 3, 2545 (except from the prologue), Reach, obveously, was in 2552. the Battle of Pagasi was the first battle for the 1st Spartan III class. only 2 "recorded" survived the battle as the Covenant Facility went to fry.
so in the end, the Cannon still has room for this game, therefore. Solid Cannon.