Short answer
HubSpot's KB: Service Hub Professional gets one knowledge base with up to 2,000 articles; Enterprise gets up to 25 with up to 10,000 articles. Creating one needs an assigned Service Hub seat.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Plan categories before writing so articles land in the right place.
3. Related
Categories and tags and multiple knowledge bases.