Does it get full sun the other half of the day? If so, I have grown them in full sun 1/2 days just fine, they like their roots in the shade and their tops in the sun! Janet

I have an arbor that size with two on each side.

Most of my Clems are not in full sun. I don't have much full sun due to neighbor's trees.

I made sure they were the same pruning needs and they are all smaller varieties.

I have not had them planted a long time, but so far, so good.

I did visit a mature garden last year, several times, a lady who was growing Daylilies and Clems commercially, she had Clems planted together that closely all over the garden. She had multiple 4 x 4 posts with metal fencing wrapped around them with 2 and sometimes 3 Clems on each one. There was a birdhouse on top of each one and there were at least twenty posts in the garden. Maybe more. She encouraged me to plant multiples together so I did.

First, you can have clematis 18", but that would be a limit, I think. Put them at the far corners to give them as much space as possible. After 4 or 5 years the mass of one plant can get fairly large, 12-15" maybe.

Second, I would stick with one pruning type, and frankly 4 type 3's (prune to the ground each year) would be much easier, as you just clean up the whole arbor each spring. These types tend to be hardier and more vigorous.

As for partial shade (or full then then some shade?), Chalk Hill has a list on their site of clems that do well in some shade. Remember to try to combine interesting bloom shapes/sizes as well as colors! Then post pictures, of course.

Here is a link that might be useful: Chalk Hill's partial shade list