Just adding to above answer, when there are multiple knowledge bases to be hidden or visible .
In the newer versions , we can select which knowledge bases are displayed in a portal ,if no knowledge bases are configured, users can view articles from all knowledge bases in the portal.
Apart from User Criteria, we can control the visible Knowledge Bases via the Portal record under All > Service Portal > Portals .
Edit the Knowledge Bases related list (enable it if hidden), and move the required KBs to the selected list.
Here is SN docs for reference :
