Where does company ID come from?

I'm working on migrating to v2 endpoints. Through the OAuth company grant flow I'm obtaining a company GUID, but all the v2 endpoints also require a company ID—Which seems redundant. The documentation doesn't ever specify where I'm supposed to get this from. Obviously I can call /v2/companies, but will there only ever be a single item in that list for a given GUID or do I have to handle the possibility of multiple company IDs for a single GUID?