A user in Web TimeSheet can only have one supervisor at any given time. Historically there is no limit to the number of supervisors a user can have but there can never be more than one supervisor, for the same user at the same time.

If you have a requirement for the TimeSheet/Time Off/Expense to be approved by multiple supervisors. The best way to accomplish this would be by creating approver types and assigning the respective supervisors as specific users. Select Override approver's permissions and ensure that Approve, Edit and View All options for TimeSheet and Time Off are selected. Once the approver types are created, you can go ahead and create an Approval Path for either TimeSheet approval, Time Off approval or Expense approval and assign the created approver types to the approval path. Please note that a single approval path can support up to 5 different approver types.  The newly created approval path needs to be setup at the user's profile for the approval of timesheet, time off or expense.

Follow the attached screenshots for more details: