N.Y. Town Law § 10
Towns are divided into classes according to population, as shown by the latest federal or state census or enumeration, as follows:
First class. Towns containing a population of ten thousand or more, excepting towns in Broome and Suffolk counties, and excepting also the town of Potsdam in St. Lawrence county , and such other towns as may determine to become towns of the first class pursuant to the provisions of this article. Every town in the county of Westchester shall be a town of the first class.
Second class. Towns containing a population of less than ten thousand, excepting towns in Westchester county and such towns as shall have determined to become towns of the first class pursuant to the provisions of this article. Every town in the counties of Suffolk and Broome and the town of Potsdam in the county of St. Lawrence .
N.Y. Town Law § 10