Not a licensing expert however, as I understand it, the approver role is a sub-role under Business Stakeholder. You can give someone approver role without stakeholder but the license cost is the same. Business Stakeholders, generally, are able to approver records, view dashboards, view records that are theirs as well as those belonging to others, and they can add comments on records. Your contract order form should spell out the capability under the "subscription products" section as well.
Since you mentioned Requestors, they can only view and add comments to their own records via the service portal and do not have ability to approve or look at other's records.
