All together, Brenda and her friends make a group of 1 + 9 = <<1+9=10>>10 people
If they each want to eat 2 pizza slices, there should be 10 * 2 = <<10*2=20>>20 slices.
Since a pizza has 4 slices, they will have to order 20/4 = <<20/4=5>>5 pizzas.
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