Praetorian Bands
Note 1 from Chapter 5 of the Decline & Fall

They were originally nine or ten thousand men (for Tacitus and Dion are not agreed upon the subject), divided into as many cohorts. Vitellius increased them to sixteen thousand, and as far as we can learn from inscriptions, they never afterwards sunk much below that number. Lipsius de magnitudine Romana, i. 4.