With this cooler wetter spell of weather we are having then it is a good time for plants to settle in. Make sure there is some drainage at the bottom and then general purpose compost should be fine. How many in a pot ? A good number ; dont space them out too widely and it will make a super display in due course.

when you say drainage ..there are holes in the pot bottoms or do I need to put anything else in too? thankyou.

I like to use a few bits of broken terracotta pots but some use polystyrene chunks or small stones or gravel. It helps the pot from becoming waterlogged and lets water drain a bit better.

OK thanks a lot for that :)

Re how many plants per pot:
What types of Asters and Sweet Williams are they--dwarf, or full-sized?

they are full sized :)

In an 18 inch pot, I wouldn't put more than 7 plants.

ok thanks:)