Sorry, I don't have the code right there, but I can tell you that it's located at the beginning of the rules(md).ini file. Somewhere in the [General] section or a little farther.
Just in case this gets looked up later:
[CrateRules]
CrateMaximum=255 ; crates can never exceed this quantity
CrateMinimum=20 ; crates are normally one per human player but never below this number
CrateRegen=2.0 ; average minutes between random powerup crate regeneration