How many orcs poured out of the Orcgate when it was discovered?
Here's what the lore says, drawn directly from the indexed sources:
-1076 DR * The Orcgate Wars begin when the Orcgate is discovered by the orcs after five years of remaining dormant. Hundreds of thousands of them poured out and engulf northern Mulhorand and Unther. The Mulhorandi enlist Nar, Raumathari, Rashemi, and Sossrim mercenaries to aid them in the battle against the orcs.
Source: -1076 DR
-1069 DR * The Orcgate Wars end as the orc horde and their gods are finally defeated in the Priador and scattered throughout the region. * Following the war, the Nar, Rashemi, Raumviran, and Sossrim mercenary tribes make their way home. * The Nars establish a series of small kingdoms over the course of a century. The greatest is Ashanath by Lake Ashane.
Source: -1069 DR
-1075 DR * The first battle of the Orcgate Wars occurs. * The population of Vandal Station is reduced to a handful of stubborn hangers-on.
Source: -1075 DR
-1081 DR * A band of rebellious Mulhorandi wizards open a portal to the orcs' homeworld using Imaskari magic. This eventually results in the Orcgate Wars five years later. * The rebellious mages of Mulhorand and Unther are defeated and captured. * The Theurgist Adept Thayd is executed for starting a rebellion in Mulhorand and Unther. He prophesies the decline of both empires.
Source: -1081 DR