What percent of e coli genes are necessary for survival?
About 10% in rich media.

There are lots of papers that try to answer this question. The basic way you do it is you knock out one gene at a time, either in parallel experiments or (more common nowadays) in a pooled experiment, and figure out which knockouts survive and which don't.

The most recent study counts 358 essential genes (out of about 4000 genes in E. coli). http://mbio.asm.org/content/9/1/e02096-17.full . Figure 2 compares a few other papers. 

The exact answer will depend on the exact conditions you grow the cells in (if you have fewer nutrients or other stressful conditions, more genes will be essential).