In Euro-J, the middle seat is kept empty on Airbus planes, but on the E190, there is no middle seat.

I have been on several flights where the plane has been very full in Y, and the only empty seat (or one of the very few) has been next to me. Then again, I was recently on a longhaul where I had booked 21A, 21C was taken as well and the Y/Y+ cabin was less then half-full. I'm glad I asked, because the check-in agent was able to move me to 22D and block the whole four-seater for me.