New novelization and prequel of the upcoming Anime movie, 怪獣黙示録 (Kaiju Mokushiroku: Monster Apocalypse) tells that Zilla? and babys joined global monster attacks.
Not being more tough as a monster, it is possible to defeat by volley of normal weaponry. However, matured individual is fast and agile, and babys are too strong to defeat by bare hands. Reproducing asexually, drastic hunts not miss a single eggs or hatchlings are required for ultimate defeat.
Not only the overwhelming numbers, they display highly evolved group tactics where babys move as decoy to support surprise attacks by adult. As a pack, they are very dangerous, becoming formidable foes even to armored forces in street battles.
First encounter was by an army to take back Europe from monsters nearby Rouen, defeated the monsters by volley of Masers and Railguns. Characteristics such as being bipedal and fins caused humanity to misidentify as Godzilla and the world was overjoy to hear the defeat of Godzilla. Revealed to be a different race soon after, Godzilla of France was reclassified and processed as Zilla, but ultimate defeat is very severe because of breeding capability and group tactics, forced humanity to consider the usage of nuclear weapons.
Not sure how many individuals appeared as a whole.