Excerpt from the DB2 plugin guide.
IMPORTANT: For all backup jobs, set this field to 1 or 2. If you enter a different number, the
plug-in limits the number to 2, and displays the following message upon completion with
warnings: Backup Option 'Number of Sessions' = 'n' changed to '2'
for 'FULL' or 'INCREMENTAL' or 'DELTA' backup
User guide:
https://support.quest.com/technical-documents/netvault-plug-in-for-db2/12.0/user-guide