？
You're saying you don't have enough friends with benefits?
Literal
Friends-with-benefits [subject-が] insufficient [quotative-って] [explanation-いうの]?
セフレ is a crude contraction of セックスフレンド ('sex friend'), meaning 'friend with benefits' — a casual sexual partner without romantic commitment. っていうの is the casual contraction of と言うの ('is that what you're saying?'), a rhetorical echo question expressing incredulity. The sentence is short, confrontational, and loaded with slang and intimacy.