It depends on what numbered group your plant is in. You need to know the name. Always keep clematis labels/names or make a note in your gardening book about the group and pruning time. I have only 3 and they have all been shortened before the growth gets too active. They are growing away well now and will not get 'leggy' with no blooms lower down and everything at the top.

If you have not got the names, do you know when they would normally flower?

If yours flowers around June or July through to September, with nothing before then, they're the ones which should be pruned down to about 8 inches from the ground in mid February. If they flower earlier, then best with no pruning. If you're talking about a species clematis, such as montana, these aren't usually pruned at all, although can be done after flowering to restrict size.