Number the genie got wrong? (8)
I believe the answer is:
eighteen
'number' is the definition.
(eighteen is a kind of number)
'the genie got wrong?' is the wordplay.
'got wrong?' indicates an anagram (I've seen 'got wrong' mean this).
'the'+'genie'='thegenie'
'thegenie' anagrammed gives 'EIGHTEEN'.
(Other definitions for eighteen that I've seen before include "XVIII" , "Number of on-field players in an Australian Rules football team" , "Age of electoral majority in UK" , "Twice 22 across" , "Treble six" .)