Got my cata invite today, and can confirm that beta accounts (or other ptr accounts) do not count toward the 8 total, so you can still get into beta if you have 8. As far as I could see anyway.

Right now have 9 accounts showing in my battle net, two of which are flagged as PTR, one for the regular PTR that is not going on right now that got created accidentally as i was clicking around, and one for the cata beta that appeared automatically today when I got into the beta.