Hi Qi;
If you are talking about multiple level of approval then it is related to whole configuration in the backend. It depends on how the workflow of that particular leave type is handled.
If you mean and option of selecting multple approvers then I dont think so it is available in the fiori application My Leave Request. It only gives you an option to select an approver. Could you please provide the link where you have seen the option to select multiple approvers.
Some useful links regarding the same are as below
Configurations for enabling Multiple approvers for Leave Request
Regards,
Sarbjeet Singh