The following sentence contains

it is not right to make a promise unless one is sure he can fulfill the promise.

one main clause
two main clauses
one main and one subordinate clause
one main and two subordinate clauses

It is not right to make a promise unless one is sure he can fulfill the promise.
This sentence contains one main and two subordinate clauses.
The main clause is It is not right to make a promise, and the two subordinate clauses are unless one is sure and he can fulfill the promise.