User blog comment:B214/Volume 23 discussion blog/@comment-31764731-20170421211419/@comment-79.25.167.4-20170421225548

As many as they want, there is no limit. The peerage and the harem are not related, it's Issei who wants a harem both out of his peerage and as his peerage.

Rias, Akeno, Koneko, Kuroka, Kunou, Bennia, Le Fay, etc.. are not and won't be part of his peerage, but they will still be part of his harem.

Anyway Issei said that he will accept any beautiful girl who will love him in his harem, and considering that Dragon attract women (and the stronger they are the more they attract), and Issei's lifespan he could have even thousands of wives in his whole life.XD And, speaking clearly, he wants to be a Harem King, and considering that in the Underworld many men have harems (even Azazel had many harem in his long life, as he said), he should have a incredibly amount of lovers and wives to be considered the Harem King.

His peerage will be only a part of his harem.