To get a good display quickly in a pot, I would always plant two, because it will be a while before they mature and even then they will not be as large as clematis planted in the ground IMO. To get lots of stems, I would cut off the stems I have at about 12" which gives the plant a setback and delays flowering but ensures that there are plenty of stems from the start. As has been pointed out, clematis like cool roots, so if they are in the sun IMO they really need other plants in front of them, if in a pot, since shading them with a stone or something can be ineffective if the stone itself gets hot. They should be planted with their crown a bit below soil level, because this helps prevent clematis wilt. They may well need to be gently tied in - they do not twine and although they do have a gripping mechanism (not sure what it is called), it isn't always effective enough. The stems are fragile and handling them can cause them to die back, so be careful. They need plenty of watering and feeding for lots of flowers, although they are tough and can survive mistreatment. So there's an argument that you need four!! The other thing is that they will probably flower at the top only if the stems are allowed to go straight up and really need to be encouraged to climb diagonally up the support (if possible). Hope this helps.