Will there by an ES 6.6 release or is 6.5.x the last release before 7.0?
I'm curious: why do you want to know this?
As it stands, the 6.6 branch has already been cut, and a number of PRs labelled for v6.7.0 have since been merged. Read into that what you will, but remember that no release is certain until it's actually released.