Hi Pauline,
Thanks for reaching out on this.
For more information see this link:
https://techdocs.broadcom.com/us/en/symantec-security-software/endpoint-security-and-management/endpoint-protection/all/licensing-v15883016-d31e6/Overdeployed-or-expired-licenses-in-SEP-and-SES.html
NOTE: You can reclaim license seats that are not used anymore.
You enable SEPM to remove clients that have not checked in within the past 24 hours, which makes these license seats available. If the number of installed clients is more than the license seat count, then this procedure does not resolve the issue. In that case, you must purchase more seats.
Best,
Russ_V