Are numbers adjectives? Is two or three an adjective? Is several or many an adjective?
Numbers are not adjectives.
Numbers are numbers. If a sentence said that there was 20 fires in one night, all the numbers do is tell the reader how many fires there were in a night.
Consider the following:
He saw trucks.
He saw the truck.
He saw a truck.
He saw one truck.
He saw two trucks.
He saw several trucks.
He saw many trucks.
Notice that in these contexts, the numbers do not describe an attribute of the truck or trucks, but function more like an article.
The modern term for such a role is determiner.
In other contexts numbers can be nouns.