Assuming you are talking about local users for SMB (CIFS), then yes there is a maximum number of local users. You can see this for each platform on NetApp Hardware Universe (HWU) https://hwu.netapp.com/
Search for the fields:
- Maximum number of local users
- Maximum number of local groups
- Maximum number of local group members
Example (FAS 2720 w/ ONTAP 9.14.1)
- Maximum number of local users: 5,000 per cluster
- Maximum number of local groups: 10,000 per cluster
- Maximum number of local group members: 30,000 per cluster