'Half-Close' figure of Data Communications and Networking, 5/e
The FIN segment consumes 1 sequence number as it does not carry data. ACK does not consume a sequence number. The FIN segment is sent by the client process. The server process receives a FIN segment but does not send a FIN segment as the connection is not closed, since it is a half-close process. Server process send only an ACK segment to acknowledge receipt of FIN segment from the client. Hence, the sequence number decreases by 1 and becomes $y-1$.
Contrast this with the Three way handshaking, where the sequence number is $y$ because in that process, the server sends a ACK segment as it acknowledges receipt of FIN segment from the client and sends a FIN segment as it terminates the connection.