How Many Millions of Monarch Butterflies? Scientists can only estimate how many millions of butterflies are in a monarch colony. They say a colony contains between 10 and 50 million monarchs per hectare of forest. One hectare is 2.47 acres, about the size of two and a half football fields. Every winter, scientists visit the 12 sanctuaries and measure the area of forest covered by monarchs. The colonies covered a total of 1.13 hectares of forest in December 2014. How many millions of monarch butterflies were there? A monarch colony covers the trees in the El Rosario monarch sanctuary. A monarch colony is a large cluster of butterflies. A monarch sanctuary refers to the whole forest within which the colony moves. Try This! Monarch Math Estimate the number of butterflies in 1.13 hectares of forest. If there are 10 million monarchs per hectare there would be ____________ million monarch butterflies wintering in Mexico.   If there are 50 million monarchs per hectare there would be ____________ million monarch butterflies wintering in Mexico.   Based on these estimates, there are between _________________ and _________________ monarch butterflies wintering in Mexico.