While you must select at least one user from the Workflow Approvers setting, there is no limit to the number of users you can add to it. For example, you can select just one user, which will only add one stage of approval, or five users, which will add five separate stages of approval.
If you've saved a draft offer where you defined approvers, and an approver no longer has an "Active" employment status, their name is no longer selected in the Workflow Approvers setting the next time you open the Create/Edit Offer form, and they can no longer be returned in the employee search.
The only way to change the order of users in the Workflow Approvers setting is to remove the ones you want to change (by clicking the x next to their names) and then to add them again in the correct order:
Administrators can configure more than one workflow for defining approvers. For example, workflows might have different notification content for offers of employment or promotions.
In this case, when you switch between workflows in the Select Workflow setting in the same offer, the approvers that you already selected for the previous workflow carry over to the next workflow you select. However, you can edit approvers as needed.