I used 2x4’s between the posts to hang these pickets.
It's just simple math. The boards are approx 5.5" wide. 75' is 900 inches. 900 divided by 5.5 is 163 boards. It will probably be slightly fewer because of gaps in the boards. And you'll likely get a few bad boards, so get at lest 10 extra and you can take them back if needed.
Call the store .
I think they are Canadian pine.
They won't hold well. Use fencing nails, they are galvanized and have ring shanks to hold better.
We buy a lot of this type fence pickets, for our farm, we not only use them for fences we use them for siding for our out buildings, we always get the best price by going into Home Depot and getting quotes from the person in charge of selling large material lots to builders.
it should be, just go on line and check or call the store and they will let you know.
240 pickets per pallet/