No enough lights, IMHO.
It really depends on the diameter of PVC you use for the arch ... and the spacing of the lights your using.
I'd recommend trying it out and see how far your lights go.
I tried to wrap them loose (for my PVC spinners) and found it was a pain (difficult, not good, moved a lot).
It's much better to wrap them tight.
Some PVC sizes and light combinations produced lights that did not distribute well (light bulbs in same place each time around) ... so find a combination that spreads the lights around nicely. (mine went diagonal in a spiral around the PVC eventually) ... I found my walmart lights wrapped nicely, but the onces from lowes were difficult because they repeated on the PVC size I was using. I would start and end each wrap with a larger zip tie. I used a PVC cap (more $$) to hold the wires on better on one end.
If you want to stretch out your lights, use a smaller size PVC pipe, but then I'd think they would need to be fairly short or they would become unstable.
TO model the arch, go into Google Sketchup (free) and draw an arch with a 10' arch.