Is customers a possessive pronoun

No, the Word customers is a plural noun, the plural form of the noun customer.

A noun is a Word for a person, a place, or a thing.

The noun customers is a Word for people.

A possessive pronoun is a Word that takes the place of a noun that belongs to someone or something.

The possessive pronouns are: mine, yours, his, hers, its, ours, theirs.

Example:

A number of customers complained about the lighting. The problem with the lights is theirs not mine. (theirs=the problem of the customers)