Myriad - 10,000 (from greek 'murioi')
10,000 men - myriad men
10,000 of men - myriad of men
Of course, in modern English usage, it is often not used to mean exactly 10,000; just the way 'dozens' and 'hundreds' get used loosely, this has now come to simply mean 'a great many' in most cases.
The form remains the same, though. 'Myriad' should stand alone without 'of' following.
Hope that helps.
N.B.: Also, with regards to the question, 'myriad' can also be used to refer to something with a wide variety of elements/parts - "the myriad political scene" from OED - Here you see that 'political scene' is singular. So you could say:
The myriad things in his office - meaning 'the many items in his office.'
or something like:
The myriad apparatus/paraphernalia in his office - meaning 'the wide variety of items'