where to set the number of total leave days that an employee can take? you see the values in red underlined. where can I set these values? in my case, it shows "0 remaining out of 0 days"

Consider this example ,

Create an allocation for 20 days for an employee,

Then it will show "20 remaining out of 20 days" it means that an employee has 20 days of allotted time off,

Employee requests and is granted 8 days of time off, then message will change to "12 remaining out of 20 days". ( 20 days allocated leave - 8 days already took = 12 days remaining)

The remaining days in the Time Off module will decrease when an employee requests and is granted time off. The number of remaining days will be updated based on the duration of the time off that has been taken.

In your case "0 remaining out of 0 days" means 0 leaves allocated and 0 leaves remaining.

To set number of leaves an employee can take ,
Go-to "Time-off/Approvals/Allocations" and Allocate leaves for the employee, These allocated leave will be the total leave days that an employee can take.

@Imran Ahmed
Number of remaining days will appear after approve the allocation