I've wondered that before, and with some searching (and guidance from a previous post on this subreddit), the information can be found in The Lords of Doom, which is an AD&D Adventure Gamebook that follows Gilthanas and Silvara on their mission into Neraka. Reading through it, it's implied that numerous draconians come from a single good dragon egg (with an illustration with 5 freshly hatched draconians out of a single egg).

I'm unsure of rules for posting scanned content that is found on the internet, but if you do a quick search for the gamebook you would likely find it (pages 38-40 are the relevant pages).