Amount, distances and number can be taken as single units if used as subjects.

1. Two is enough.
2. Fifteen ringgit is what I have to pay.
3. Sixteen kilometres is a long way to talk.

But,

1. Two watches have been stolen from her locker. (2 units)
2. There are 16 kilometres more to cover. (16 units of distance)
3. There are 365 days in a year. (365 units).