Well, you are speaking to a person who packs them in when it comes to clematis so I would go for at least two. I would be tempted to put one on each side of the obelisk but like I said, I push the limits on most things related to clematis.

I was thinking one to a side, but whether it be square (obelsik) or triangular (teepee) there is n need for one in the crner. So three on a square or two on a triangle would be alright? Spread them out as much as possible? I am hoping that shorter clems won't need quite as much space...

I forgot to say "Thanks!" :o)

It really depends on what Clems you choose.