What percentage of cumulonimbus clouds create lightning?
I've done some research studies about the flash rate in various storms. My paper is now in review, but here is some info from this paper:

We have computed measurements for supercell and moderate thunderstorms, counting CG+ discharges. There are 24 times more discharges in supercells than in moderate thunderstorms. Supercells on average had better thermodynamic conditions to develop into a huge storm cluster.

The only help to answering your question is that Cb clouds are not similar to each other. But what I can tell you is that Cb lightning in my studies for CG+ discharges occurred once a minute, whereas for a supercell this number is equal to 24 per minute.

I studied thunderstorms from 19th of July 2015 and 17th of April 2016 over Poland.