User blog comment:B214/Season 3 confirmed!!!!!!/@comment-3965848-20140618064204

If there was to be a season 4 (fingers crossed everybody) the first half would be Vol. 7 right, I think for the second half they show at lest two short stories and that time Issei and Rias were at the Ruins of Connection. That way future seasons will turn out like this:

Season 5: Vol. 9 & 10

Season 6: Vol. 11 & 12