I always took it to mean the Enterprise was the 40th ship. They were supposed to join the battle, but it was already over when they arrived.
From http://www.chakoteya.net/NextGen/175.htm:
HANSON [on monitor]: Your engagements have given us valuable time. We've mobilised a fleet of forty starships at Wolf three five nine, and that's just for starters. The Klingons are sending warships. Hell, we've even thought about opening communications with the Romulans.
Hanson differentiates "you" from "we" (boldfaced), so it's a stretch for Hanson to be thinking of the Enterprise as a part of the fleet of starships that have been mobilized at Wolf 359.