Is it true i should cut my butterfly bushes back to about 12"s

My vote s no. It can actually kill the bush in some cases. I once had an inexperienced "gardener" and it killed the poor plant. Here is a link which provides wonderful tips!

https://www.growingagreenerworld.com/pruning-butterfly-bushes/

Yes, it will fill it out more but it’s too late now. I’m not sure but Spring flowering bushes like Quince, Lilac, Forsythia all get trimmed when it finishes blooming. I don’t know when is proper time to trim butterfly bush. Most things get only top third taken off at a time unless you are looking to severely reshape the growth. Good luck.

No,it is not necessary to prune them all the way back.Next season cut out the dead wood

https://dengarden.com/gardening/planting-butterfly-bushes-buddleia

http://urbanbutterflygarden.co.uk/how-to-prune-and-shape-buddleia-butterfly-bush

We do in zone 9.

I cut mine back severely. It's scary...but in the spring? They grow beautifully !

That's about the length we go for, 12 inches.