Answer: The following sentence contains one infinitive.
Step-by-step explanation: An infinitive is a verb form that consists in the particle "to" and the base form of a verb. It can function as a noun, an adjective or an adverb. Moreover, an infinitive generally appears in an infinitive phrase. In "It is not right to make a promise unless one is sure he can fulfill the promise" "to make" is an infinitive since it has been formed with "to" and the base form of the verb "make". Furthermore, this infinitive is part of the infinitive phrase "to make a promise".