City have always had a huge "latent" support. I remember very clearly the early years of the Mercer era. We had been dogshit for years, and regular crowds had dropped to something like 10,000-15,000, with occasional dips below that. As soon as we got a half-decent side, the people came flooding back. I remember a night match against Norwich (Still in the 2nd Division). 34,000 rocked up. Paddy Crerand had actually bet Allison that City would never have more than 30,000 in the ground again. He had to pay up!
Before long we were getting 50,000 - 60,000 for the big matches. And there lies the difference. Back then people would turn up on a random basis, 99% of the time paying in on the gate. So the crowds varied hugely. Funnily enough, it wasn't always the "big" teams that got the biggest crowds. I remember a cup replay against Newcastle when there was barely room to breathe in the Kippax, it was that full. One of the few times I ever stood at the back - there was no choice.
City's current regular support is probably bigger than at any time in our history. But I wouldn't mind a small bet that there's at least as many again who would appear for big matches if the capacity was there and you could just wander down and pay on the day. That's why I always say "build it and they will come." Because with this side playing incredible football and winning trophies by the sackful, the sky is the limit.