User blog comment:Xilinoc/Ch. 590 - Marching Out the ZOMBIES/@comment-24546844-20140723141841/@comment-11051098-20140723235836

Mayuri was in Fake Karakura Town before they switched it with the real Karakura Town, I would guess he picked up every body he found. Also, who knows if he has more Arrancars than just those 4? They could just be the introduction.