Gardening and Horticulture - petunias

I am a beginning flower gardener. I am trying to start petunias from seed. When it says on the back of the package that "Flowers are single", what does this mean? Does it mean one flower per plant? I would like the kind of plant that grows many blossoms per plant. Also, any advice on starting petunias from seed would be appreciated.
(I'm growing Petunia hybrida--Viola Blue)

I'm planning to grow nasturtiums in a pot and a hanging basket. I read that nasturtiums like sandy soil. Can I add course sand-box sand to commercial potting soil to achieve a good soil (after sterilizing the sand)? If so, how much sand? (I sterilized the sand by baking it in the oven for 1 1/2 hours at 400 degrees. Is that acceptable or am I crazy like my kids said?) (Nasturtium--Empress of India)

flowermom, The "single petunias" refers to the amount of pedals on a flower. In other words a single petunia would look more like a Daisy with all of its pedals in a circle. The "fully double" Petunias will be frilly and fluffy like a carnation. Most single flowers look little like their double cousins.
I don't know the absolute rule about using builders sand in the garden but was told a long time ago not to use it. I was advised to get river sand and that is what I have always done in that we have 3 rivers here....Mike