they spawn once after the flower blooms, but it's not a 100% chance that there will be a butterfly when it blooms. after that, no more butterflies will spawn unless a friend shares a butterfly with you. your best option is to harvest and plant more flowers as soon as you can, while leaving some fully grown flowers incase a friend decides to share with you.
Visit my dream towns!
Angie from Maplerow, Ranae from Wayfield and Olive from Yorkwood