▁em peror ▁T iber ius , ▁un ited ▁the ▁Urban ▁C oh ort s ▁with ▁nine ▁Pra et or ian ▁C oh ort s , ▁dispers ed ▁at ▁that ▁time ▁throughout ▁Italy , ▁in ▁one ▁large ▁camp ▁situated ▁beyond ▁the ▁Serv ian ▁Wall , ▁on ▁the ▁Es qu il ine ▁Hill , ▁the ▁Cast ra ▁Pra et oria . ▁ ▁For ▁the ▁ 2 nd ▁century , ▁calculations ▁from ▁lists ▁of ▁significant ▁dem obil is ations ▁suggest ▁an ▁increase ▁in ▁size ▁to ▁nearly ▁ 1 , 5 0 0 ▁men ▁per ▁coh ort ▁( per haps ▁a ▁dou bling ▁of ▁ 8 0 0 ▁( since ▁V esp as ian ), ▁probably ▁organized ▁in ▁ 2 0 ▁centuries ) ▁under ▁Com mod us ▁in ▁year ▁( 1 8 7 – 1 8 8 ) ▁or ▁under ▁Sept im ius ▁Sever us ▁( 1 9 3 – 2 1 1 ), ▁which ▁matches ▁the ▁probable ▁numbers ▁of ▁effect ives ▁for ▁Urban ▁C oh ort s ▁during ▁the ▁time ▁of ▁Cass ius ▁D io . ▁These ▁figures ▁suggest ▁an ▁overall ▁size ▁for ▁the ▁Guard ▁of ▁ 4 , 5 0 0 – 6 , 0 0 0 ▁men ▁under ▁August us , ▁ 1 2 , 8 0 0 ▁under ▁Vit elli us , ▁ 7 , 2 0 0 ▁under ▁V esp as ian , ▁ 8 , 0 0 0 ▁from ▁Dom it ian ▁until ▁Com mod us ▁or ▁Sept im ius ▁Sever us , ▁and ▁ 1 5 , 0 0 0