I assume that the 787 is (self)certified using known knowns as certification rules, but then I'm curious what and how many unknown unknowns regarding plastic electric jets are out there, waiting to either be discovered or found out the hard way...
Every single plane that enters commercial service enters under these identical presumptions.
Every single plane eventually turns up the unkown unknows in its service life, some quicker than others. In spite of the variables of "when" the regulatory continued airworthiness process ADs etc. is designed to maintain an acceptable level of safety.
After this level is met the rest of the story is economics to the users. So your question about how many, is what makes investing risk in a company and has little to do with flight safety.