I don’t know where the number 2501 comes from. (There might be a meaning in chinese numerology I’m not getting — that’s not really my forte.)
The explanation that 2501 is an insider trading system is on page 269 of my copy of the manga (Dark Horse’s first 1995 English edition). It might occur on a different page number in other editions. 2501 explains it himself when Kusanagi first dives in.