Depends upon how many friends you have some of the materials filled already(low -> max fill(9), max -> low fill(0)), After that maximum 4 neighbors are needed to finish the Butterfly house. The required number of builders depends upon how many neighbours you have. For example If you have more then you need 4 builders, if you have only 5 neighbours then only 1 builder required and remaining 3 pre-filled.
Harvest crops to collect Pollen. Crop Packets produce more Pollen. Fill 10 Pollen to tend the Bee Box and chance to get a Honeycomb. To collect all Honeycomb to collect a Honey Bee Hive from Walter. And also collect all 5 Honey Bee Hive to win a Beekeeper Suit.