Possibly 50 musicians or 100 -- that guy's creative work is revolutionary (9)
I believe the answer is:
orchestra
'possibly 50 musicians' is the definition.
(an orchestra might have 50 members)
'or 100 that guy's creative work is revolutionary' is the wordplay.
'100' becomes 'c' (Roman numeral).
'that guy's' becomes 'he's'.
'creative work' becomes 'art' (artwork).
'is revolutionary' is a reversal indicator.
'art' reversed gives 'tra'.
'or'+'c'+'hes'+'tra'='ORCHESTRA'
(Other definitions for orchestra that I've seen before include "LSO, for example" , "Large body of musicians" , "in four sections" , "eg Halle" , "Large music group" .)