First of all, that looks awesome and I can't wait to see the finished animal
Check out this post I just wrote on brightness and power considerations
. I agree with Josh that 8 per arduino is the way to go.
When stringing lots of Stribe1s together in one past application, I ran into an issue of large voltages appearing across the arduino ground and last Stribe1 ground, because the thin ribbon cable wire had too much resistance. I was using 6 feet of ribbon cable in that case, however. You probably wouldn't run into this issue with 13 close together, but it doesn't hurt to be safe.
My advice is to use an externally powered USB hub.
Another issue is timing. Every additional stribe1 adds time to the loop due to sampling another analog input, and sending back and forth more data. At 8 stribe1s, it's a little bit noticeable, and may get annoying at 13. Splitting it 8 and 8 offloads more work to the computer.
If you just want a simple relationship between touch and lights, you could always have the arduino take care of drawing instead of the computer, and not have to worry about the usb-serial port slowing down the response. (coding required). Our current code is set up for maximum flexibility in max/msp.