Final answer:
The question does not provide enough information to answer accurately.
Step-by-step explanation:
The question asks how many butterflies were in the population when they started the migration.
To determine this, we need to know the size of the population at the start of the migration. Unfortunately, the information given in the question doesn't provide the population size. Without this information, we cannot accurately determine the number of butterflies in the population when they started the migration.