Board Thread:Warhammer 40k General Discussion/@comment-69.143.134.163-20150419161351/@comment-80.114.86.73-20180125154823

Yes, but that implies that one primarch already fell to chaos. In turn, it would make no sense for Manus to suggest he could have 15 instead of 13 (those were the correct numbers btw, not nitpicking but I found the source), if one of them already fell to chaos.