I think two in a 15-20" pot would be more than enough. Creeping rosemary gets big, and you may have to repot it late this summer. Plant them separately, off-center, so they will reach the edge of the pot more quickly and also you'll be able to separate them later.
Wish I could find 6-packs of creeping rosemary!

Richters sells "Huntington Carpet Rosemary" by the 12-pack. The 12-pack plants are small but they grow and it is a great way to get a number of an herb plant that you wouldn't want to start from seed. The plug trays (120 plants) are a great deal too if you want to plant a whole area into something. There are minimum numbers to buy on these so that can be an issue if you only want one but get a friend to go in on something with you.

FataMorgana

I have seen a single creeping rosemary cover an area of at least 2 metres in diameter, with equally long trailers drooping over rocks in a tiered garden.

Over to you.

Be sure to root it and share with people who aren't as lucky as you to find 6 packs..
Blessings from the Garden
MLCH

Thank you for your help everyone. Maybe since i have six, i'll try planting one in a pot, and two in another pot and see what happens. Now to find someone who wants the other three.