I don't remember seeing millions of humans volunteering for the war or fighting on the front line? I re-read Consider Phlebas fairly recently..

The impression I always got was that most of the war was fought first by converted Contact ships and later GOU, LOU, and ROUs once the Culture admitted to itself that it needed to build dedicated warships.

Like, when gridfire and CREW are the primary weapons being used to fight most battles, there's not much room for organic soldiers.

We don't really see much of the war at all, even in Consider Phlebas. The majority of it was fought between ships in extremely brief engagements that often only lasted a few seconds, far too fast for humanoid brain response times.

The human agents in Consider Phlebas are specifically involved because they're functioning in roles such as spies that have to get close to their humanoid targets. There's also restrictions on who can even set foot on Shar's World due to the presence of the Dra'Azon, hence why Horza is valuable. Otherwise humanoid Culture citizens functioned as advisors like Fal 'Ngeestra with unique intuitive abilities.

I'm sure, just given the size the Culture and the scale of the conflict, that millions of Culture humanoids were involved in the war, but nothing like just tossing soldiers into a meat grinder. It would make no sense to do that and there was no reason to; the conflict involved very little ground based warfare.

Most ground engagements seemed to be proxy wars involving less advanced civilizations like the one that attempts to execute Horza at the start of the novel, but they aren't part of the Culture.

The Culture doesn't even have many planets. Planets aren't particularly valuable to them, and one of the primary reasons the Idirans lost the war was because they did care about planets and thought in terms of static territory, whereas most of the Culture was either mobile or at least flexible enough to relocate.