Didi
You could put about 4 or 5 in a 10 in pot. Place 4 around the rim of the pot and one in the center. Pinching is taking the growth tip off basically the first tiny leaves. That will help them out a great deal.

When I do mine in containers I space them a lot closer than I would in the ground & they always do beautifully. I think it's because being container bound they get more fertilizer & water than my inground plants simply because they need more.

I just did a whisky barrel planter with 1 Astilbe, 2 upright Fuchsias, & 11 multicolored Impatiens - all spaced just a few inches apart.

I use fairly large 16" and 20" pots for my impatiens on my deck. I use 80% homemade potting soil and 20% composted maure. I even put unfinished compost on the bottom of the 20" pots if I'm a little short on soil.

But...I'm a frugal gardener... I only put 4 plants in each pot. I buy them at Wal*Mart @ 6 for $1.62 or 27cents each ...so each of my 12 pots has $1.08 worth of plant material. By the first of August they all look like big ice cream cones of color.

I lop off about an inch of each plant just before planting. They fan out and bloom like crazy. Last year the 20" pots had plants 18" high and cascaded almost to the ground...

I just keep 'em wet and give them a very light liquid fert every 10 - 14 days.

I really seem to get a lot of "bang" for my buck doing it this way

Pete in Iowa

I just shove um in there to make um fit. They get huge and look wonderful. Not to worry the more the marrier.