Great in number (13)
'great in number' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for legion that I've seen before include "Host" , "Large unit of the old Roman army" , "Roman military formation" , "Many soldiers" , "Roman military body of men" .)
(Other definitions for multitudinous that I've seen before include "Profuse" , "Occurring in great numbers" , "Large numbers" , "Very many in number" .)