It depends on how often you want to do laundry... If you want to do it every 3 days - then I would have 5 pants/shorts, 7 - 10 shirts (just because they don't always want to wear the same ones over & over). The reason I said a few more then the number of days is because you need one when you wash clothes & also if the kids get dirty for some reason. Younger kids might need a few more because they mess things us more & with potty training you never know what you will need. I do laundry about 2 - 3 times a week... I have 3 kids at home (2 1/2, 4 1/2 & almost 6)... my kids have a few more things, but we are also going through everything & cutting down what they have.
For my daughter (almost 6) we also have to have school uniforms, she has 3 pairs of pants, 3 sweeters, 2 jumpers, 6 short sleeved shirts, 6 long sleeved shirts & 2 belts. Her grandma got her a few pairs of skorts for her birthday that is coming up for her to wear this spring. But she also has a few dresses... the boys just need a nice pair of pants & a nice shirt when needed so it seems girls need a few more things.
Congratz on the baby on the way & such a large family :)