How many elves perished at the Battle of the Gods' Theater?
Here's what the lore says, drawn directly from the indexed sources:
-10700 DR * Third Crown War: The Battle of the Gods' Theater is fought, one of the greatest and bloodiest battles of the Crown Wars. Over 70,000 elves die as an orc horde falls upon the battling armies of Aryvandaar and Shantel Othreier. Aryvandaar is victorious and occupies the northern half of Shantel Othreier.
Source: -10700 DR
-1071 DR * The Battle of the Gods occurs. * In the Battle of the Gods, Ra is killed by Gruumsh. * The Untheric deities Girru, Ki, Inanna, Nanna-Sin, Nergal and Utu are killed by orc deities. * Gilgeam battles Ilneval but is ambushed by Tiamat in a surprise assault. However, Marduk steps in and slays Tiamat before she finish off Gilgeam, but Marduk is killed himself.
Source: -1071 DR
-10500 DR * Miyeritar suffers the Dark Disaster, and is left as a wasteland in three months. Most elves of Faerûn are left shocked and horrified, leaving a four-decade-long fearful peace amidst the Crown Wars. Though history as written by the victors says otherwise, the High Mages of Aryvandaar are responsible. * Due to the Dark Disaster, the larger part of the followers of Eilistraee—a major patroness of Miyeritar—die in the cataclysm. This blow drives the goddess and her church into a near-powerless state, from which they won't recover for millennia. * The chain of islands that would later…
Source: -10500 DR
-11450 DR * The Sable Wars: Aided by a few allies from Keltormir and Shantel Othreier, Eiellûr and Thearnytaar invade Ilythiir, aiming to reform or conquer the dark elves before they destroy any more realms. Over half their forces are wiped out by the dark elves' corrupt magic.
Source: -11450 DR