Answer is 15.
As given, this is sliding window protocol so,in case consider Selective repeat:
Sender window size = Receiver window size
and
Sender window size + Receiver window size = sequence numbering(24)
Let window size is N.
N+N= 24
N=23
window size=8.
If we consider GBN then the max window size (16-1) =15
So Max window size =15