Some eastern monarchs overwinter in Florida and remain reproductive throughout the winter. The overwintering populations in central Mexico numbers in the tens of millions. The number fluctuates each year. Monarchs enter a reproductive diapause while overwintering. They don't begin laying eggs again until spring.
Monarchs are the only species of butterfly that participate in a true migration.
They migrate south, then when spring arrives, return northward to reproduce. The location of the overwintering grounds in central Mexico was only discovered about 25 years ago. They do not become mixed up and unable to migrate as some extreme sources would propose. Monarchs stop and nectar along their journey south. Monarchs are the only species that truly migrate - that is the original southward migrants make the return trip to the north again in the spring.