Passage: The group consists of 18 main islands, 3 smaller islands, and 107 rocks and islets. The islands are located at the Galapagos Triple Junction. The archipelago is located on the Nazca Plate (a tectonic plate), which is moving east/southeast, diving under the South American Plate at a rate of about 2.5 inches (6.4 cm) per year. It is also atop the Galápagos hotspot, a place where the Earth's crust is being melted from below by a mantle plume, creating volcanoes. The first islands formed here at least 8 million and possibly up to 90 million years ago.While the older islands have disappeared below the sea as they moved away from the mantle plume, the youngest islands, Isabela and Fernandina, are still being formed. In April 2009, lava from the volcanic island Fernandina started flowing both towards the island's shoreline and into the center caldera.In late June 2018, Sierra Negra, one of five volcanoes on Isabela and one of the most active in the Galapagos archipelago, began erupting for the first time since 2005. Lava flows made their way to the coastline, prompting the evacuation of about fifty nearby residents and restricting tourist access.

Question: Of the 128 islands that make up the Galapagos, how many are considered main islands?
18 main islands