I have had success with Etoile Violette. Very dark purple blooms, and a TON of them. Rooguchi is also lovely. Also dark purple, it continues to bloom over a very long period. Never exactly covered in blooms like Etoile, but always something there. Betty Corning is also a long blooming, hearty plant. I am very fond of Ville de Lyon, which I've mistreated and it has not slowed down, and one that is newish to me, but seems like a real sturdy plant is Venosa violacea. I love the blooms and it seems like it's ready to really take over. It sems like the vitacellas really have a will to live, and I've also had good luck with the texensis varieties.
I don't think 4 would be too many for your arbor, but that will depend on which ones you choose. A lot of us plant pretty close.

Thank you for the help, I'll look for the varieties you mentioned. The Etoile sounds great!
I have noticed that a lot of the threads mention planting pretty close, I have until spring to decide.

I have Etoile Violette, Polish Spirit, Betty Corning, Venosa Violacea, Ville de Lyon, Rouge Cardinal, Prince Charles, Princess Diana. The first four are more vigorous and grow larger for me. Polish Spirit blooms far longer than Etoile Violette.

Heavily amended and worked soil has been key for me. In areas of clay with tree roots, Clems have struggled for me. In beds where the entire space was rototilled and composted before anything was planted they have taken off.

I hope that I will be more successful with the amended soil, I did not do this in the past. Hubby got a bit tired of my adding layers and tilling, but he stuck with it. There are no existing plants around the new bed so competition shouldn't be a problem.
Thank you both for your input, I drool over other peoples beautiful clematis. I am hoping to have some to show off next spring.