You can add approval policies for new cards for users, managers, and reimbursements by heading to Settings —> Approvals.
From there, click on the approval policy you wish to edit (employee cards, manager cards, or reimbursements). From there, click "Add approval layer". You can then determine the limit of a card you wish to have multiple approvals for or the reimbursement amount you wish to have multiple approvals for.
For all of these approval policies, you can choose whether you wish for approvers to just be notified or provide approval for new cards / reimbursements: