In the Land of Azurth, Goblins don't reproduce in the manner of humankind or humanish creatures, but instead they arise from pools of a viscious, green fluid, faintly luminescent in darkness. This fluid, which wells up from the depths beneath Subazurth, is called goblinic slime--or sometimes "goblin snot" in the vulgate of the Underfolk.
How many goblins and how fast they arise depends on available sources of energy and the volume of slime. Even a pool only a few inches dead and a couple of feet across, and in a cool, dark place can produce at least a few goblin larva. Deeper or larger pools, warmed by heat from the depths, can produce hundreds, even thousands. In ideal conditions, goblins wallow in their pools until virtually adult size, but where resources are scarce, they may crawl worth as tiny goblings only inches tall (1 HP) and are certainly a menace to others when their are a foot to two feet tall (2d4 HP).
Goblins are born sexless, but at apparent maturity (in terms of size), a slight majority develop male or female sexual characteristics, seemingly at random. While some goblins (regardless of sex) enthusiastically engage in sexual activity, reproduction never results.
There are rumors of remote places in Subazurth where slime pools are associated with strange machinery--hissing valves, wheezing pumps, gurgling pipes, and the like--attended by other goblins in great numbers. These are places the prudent avoid.
Learned texts disagree on whether goblin slime is edible, if unappetizing [a Constitution save DC 15 is necessary to avoid vomiting] or toxic to the unwise ingester by means of internal goblinization. If the former sources are to be believed, slime causes a rumbling in the bowels and strange dreams, but also may confer the ability to understand the native goblinic tongue (40% chance) for 1d10 days. If the more pessimistic sources are take for true, at a failed DC 13 Constitution save, goblins will propagate inside the individual within 1d10 days leading to 2d4 points damage a day and a DC 11 Constitution save to avoid death as they try to emerge.