Very numerous army I'd mustered (6)

I believe the answer is:
myriad
'very numerous' is the definition.
'myriad' can be an answer for 'numerous' (I've seen this in another clue). I am not certain of the 'very' bit.
'army i'd mustered' is the wordplay.
'army i'd' can be anagrammed to 'MYRIAD'.
Though, I am not clear how this is indicated.
Can you help me to learn more?
(Other definitions for myriad that I've seen before include "Numberless" , "A great number" , "Countless, extremely large number" , "Host" , "Large indefinite number" .)
