I have let my 12 clematis all grow without pruning last fall and this spring so that I could figure out when each one blooms and thus the best time to prune it. For those that are getting ready to bloom now, can I prune them after the bloom and have them rebloom again this year?

Yes, you can prune after they bloom--
But, even the ones that should re-bloom don't always 'feel like it', in my area anyway. Might work better in your climate.