That's like asking how many people that own last year's Corvette are going to skip this year's Impala in favor of this year's Corvette.
First, not many people will buy and use both models. And if you owned a 5, the odds of you choosing the 5C this year are probably pretty slim, since they have nearly the same specs.
I owned the first three iPhones, gave away the 4, and skipped the 4S/5/5s because I disliked the sharp-edged, delicate, slender form factor.
A coworker brought in his 5C today, and now I am planning to pick one up myself when I can get to the Apple store this weekend. It's not as comfortable as the 3G series is in hand, but it feels quite similar to my original iPhone (which is still kicking as iPod and occasionally as a backup phone).
I'm glad Apple is offering more than one design option. The only thing I want from the 5S is the camera. Hopefully I'll get lucky and next year's 5C equivalent will have the 5S camera in a curvy form factor.