I plan to plant my Passifloras by my backyard fence which is 75ft x 6 feet, considering that passifloras tend to grow and expand how many plants should I contemplate to plant and how far apart
Passiflora vines are not a dense vine - it will take many, MANY vines, planted about 2 feet apart to get the fence covered. It will be VERY expensive unless you buy fruit and start your own from seeds. I planted all the seeds from a supermarket fruit, many years ago, and got around 50 seedlings, but it takes time for them to be developed enough to plant in the ground.
My one flavicarpa filled out that size fence withing 2 years and then some. The first year it filled out about 10 feet and the next year it turned into a monster. It really depends on what passiflora and which zone your in.
If you're in a hurry plant 5, if not 2 is enough. With 5 plants you will have your work cut out for you after they take off.
They will try to take over your yard, so be prepared if your planting flavicarpas and edulis.
The picture is 1 single flavicarpa plant and shows less than half of it's total size.