A Warband is not the size of a Loyalist chapter. Read the page you actually linked to...
tens or hundreds = 1,000 members of a Chapter-size unitSome are not whole Chapters but simply warbands composed of a few tens or hundreds of Astartes drawn from the same original source and led by a charismatic commander or Chaos Lord.
50 * 1000 = 50,000 maximum, if 50 whole chapters went rogue.According to the current records of the Inquisition's Ordo Malleus, approximately 50 Loyalist Chapters or elements of Loyalist Chapters have turned Renegade in the ten millennia since the Horus Heresy.
When you link to sources,make sure those sources don't actually contradict your claims next time.