How many pies???

Oh man, this sounds like a high school math problem lol. Can you talk to the baker and get an opinion? It's tricky because some people may want multiple flavors.

This is purely my opinion, but I would get 9 cherry, 9 apple, and 7 blackberry. That would provide about 25 extra slices to allow for people who try more than one flavor. I would guess that blackberry would be slightly less popular than apple and cherry, although it would be my personal top choice between those 3 flavors.

I would personally plan for 2 pieces per person per pie, because people will likely want variety and you can’t determine what will be more popular.

As someone mentioned it’s probably best to ask the baker what they’d recommend. The good thing about pies is that they’re easy to take as leftovers. Here’s a few options (I’ve changed it so it’s 8 slices per pie because it’s easier to cut equally )

Option 1: 1 slice per flavor per person
Mostly everyone will get to try out each flavor (you might get some guests that’ll get one than one slice of their favorite flavor) and there’ll be plenty of pie for everyone. However, also keep in mind that 3 slices of pie may be a lot for guests, so be ready to prepared to take some pie home or provide some kind of container for guests to bring it home. With that said 150 guests can consume about 18.75 pies per flavor (round it up to 20) for a total of 60 pies

option 2: 1 slice per person
this is the most cost efficient option if you know your guests well enough. If you get equal amounts of pie for 150 guests, you’ll need 6.25 (round up to 7 pies) of each flavor. From there, you can play around with the number of pies if you know which pies are more popular among your guests and which ones aren’t. Also makes sense since you’re already providing s’mores so people don’t necessarily need a lot of pie. This is probably my preferred option

option 3: 1 slice per person + a little more
Initially get 7 pies of each flavor like mentioned in option 2, but round it up to 10 pies for a total of 30 pies. This will still be PLENTY of pie (~ 240 slices) to provide for those guests that want more than one slice, but not too much so there isn’t too much pie leftover as opposed to option 1