Yes, you can do this in multiple ways depending on the type of auth you use.
For local user, you can individually change the vpn-simultaneous-login attribute for that user to 20.
username testuser password xxxxx privilege 5
username testuser attributes
vpn-simultaneous-logins 20
If is an LDAP or Radius user, you can either use LDAP attribute map or Radius group-policy mapping feature to map the user to a different group-policy. Inside the group-policy, you can set the vpn-simultaneous-login attribute to 20. You can put the user in a separate AD group to differentiate him/her between other users.