Welcome to the HubSpot Community, and great question here!
It sounds like your requests to the Auto-Complete API have returned a maximum of 5 results.
I can confirm that this is indeed the maximum number of Companies returned by this API.
At this time, there isn't an option to iterate or paginate through further results and we recommend making the query parameter more specific, where possible.
You can learn more about our AutoComplete API and other free APIs in this FAQ article.