The number of spells you've cast is counted only as Murmuration's last ability resolves. For example, say you've cast two spells this turn, and with Murmuration's last ability on the stack, you cast a Blizzard Strix (a Bird creature spell with flash that may have been chosen for this example due to being an adorable owl). When Murmuration's ability resolves, you'll have cast three spells so far this turn, so you'll create three Storm Crow tokens. Caw!