It wasn nowhere near 7,000 .... MIllwall only averaged 7,000 fans in the league that season , so for that to happen they must have fetched almost EVERY supporter that they could muster!
They have NEVER brought anywhere near 7,000 to City , the section of the ground that they were allocated in the North Stand only held around 3,000 visiting fans to start with , and i think that's all the amount of tickets that ANY visiting club was allowed in the league anyway that season ... although for a cup game you got a larger allocation, as you always do.
City, for example, had almost almost 13,000 at Old Trafford for an FA Cup game one season ....