The paper deals with Suarez's concept of a number as it is put forth in his famous Disputatines Metaphysicae. Suarez's explanation is led by the standard scholastic question with regard to the nature of a number, namely whether a number is the real species of a quantity. Suarez answers the question in a negative way and his main opponents are in this respect Thomists. Suarez rejects the Thomistic concept of a number according to which a number is the real species of a quantity. Suarez's argument is based on the claim that a number is not a being (ens) per se and that is why the Thomistic thesis is not valid. According to Suarez, a number is (broadly speaking) an aggregation of units, which of itself does not have a sufficient type of unity in order that it could constitute a being per se. However, a number has some kind of unity but the unity of a number comes from the cognitive act of our reason which subordinates an aggregation of units under some concept. Only if we consider an aggregation of units with this kind of unity can there be strictly speaking (or formally speaking) a number, e.g. the number of the apostles. A number taken in a formal way then somehow belongs (but not in the strict sense of a word) to the species of a quantity.