Certified GKH Gardening Expert
It depends on the plants and the size of the basket. I almost always buy too much and have to find another spot for my surplus. You want it to look full right away but then you need some growing room, too. Purchased hanging baskets aren't put out for sale till they are full and looking good. So when you do your own, the basket may look sparse for a short time till the plants fill out. For a 10 or 12 inch basket, 4 to 5 plants should be enough (in 2 to 4 inch pots). Here are some plant suggestion:
https://www.gardeningknowhow.com/special/containers/hanging-basket-plants.htm
https://www.gardeningknowhow.com/ornamental/flowers/petunia/hanging-petunia-plants.htm