Also, how many seeds per pot?

I've grown Calendula, Alyssum, Balsam, and Morning Glory in pots.

Balsam and Calendula likes deeper pots, I'd go with 10" diameter. The problem I had with the balsam was that they got pretty tall and on windy days tended to topple over. Calendula doesn't like to dry out. Alyssum seems to like being sowed where they will live, but I had mine in a fairly shallow flower box, only about 6" deep and 18" wide, with about 6 plants in there, maybe more. Morning glory is very easy to grow, but it helps if you soak the seeds overnight and nick them at the pointed end before planting them out. Plant MG's seeds out after soil temps get consistently in the 50's, about the same time you would put out tomatoes.

Again, with a 10" pot, I would try 6-8 seeds per pot for the Alyssum and Calendula, and maybe 4-6 seeds per pot for the MG's. It's better to plant more than less, as you can either pull out the extras or move them to another pot. Beware that MG's don't like to be transplanted, and they don't really need fertilizer but it doesn't hurt. Supposedly you get less flowers and more foliage with fertilizer. Otherwise, the other flowers would enjoy some.

Also, since Gaillardia is a perennial, you won't get flowers this year for any you sow, but will have to wait until next year for blooms (unless it's a new hybrid that flowers the first year?).