As many as you can afford. They will eventually fill in....but I want my baskets to look good NOW....so I put the plants very close.
But be aware more plants means more water and more feeding. I feed about 1/5 strength liquid fertilizer about every time I water....but confess to sometimes just using the hose!
Linda C

I thought if I put the plants real close together there wouldn't be enough soil for them to grow...
Thanks Linda...I want mine to look good NOW too...and I will repot them tomorrow.
Have a safe weekend!

At the wave petunia web site, they suggest 3~4 plants in a 10" basket.
Here is the link that might be needed.

Here is a link that might be useful: wave petunia

ALso, your plants may look scraggly because they need to be pinched back atleast twice while they are growing. Pinch back the long shoots once they reach the edge of the basket,and that will allow it to fill out the center of the plant. Then after a couple weeks, you can pinch them again...and then let them grow. Wave Petunias are extremely heavy feeders and require ALOT of Fertilizer. It would be beneficial to your Waves to fertilize with every watering (every day) until they are well on their way,,then you can fertilize once per week. Goodluck !

I put 9-10 plants in a 14 inch pot and they turn out absolutly beautiful. I water them every day or every other and fertilize them with egg and miracle grow. Lots and Lots of water and sun i think is the trick. Good Luck!
8-)

I have a small nursery and put 2 petunia plants (any type of petunia) to an 8" basket and 3 though sometimes 4 to a 10" basket. By the time they start flowering they are just hanging over the side of the basket.
For fertilizer I use Osmocote 4 month slow release which seems to work just right.

I usually put 3, just so they dangle in each of the 3 spaces between the hanging arms.

I don't have any of the bigger hanging pots but the smaller ones always have a recess in the bottom, where the drain hole is actually about 1/2 inch above the bottom surface of the pot. This means that after watering, 1/2 inch of water sits in the bottom of the pot. This can cause the plants in hanging baskets to be killed way before summer is over. Since I started putting holes in the true bottom of the pot, all hanging basket plants have done so much better. A shrub pruner is what I use, and can be done while the plant is in the pot.