An excellent site for container recipes is at Proven Winners website. There are several great recipes there with wave petunias. Enjoy!

"The man who has planted a garden feels that he has done something for the good of the world." Warner

Here is a link that might be useful: Proven Winner Container Recipes

Wave petunias get huge and I'd say 3 or 4 plants would be enough for your 30" flower box. They may take more time to look full but it's worth it since the more space you give them, the bigger they'll grow.

You can plant all the different varieties together, but for flower boxes I'd go with the easy waves and shock waves, which grow more mounded. The plain waves grow really flat and the tidal waves too tall.

I usually plant petunias with zonal geraniums or angelonias.