Numbers are nouns; any number, such as 44 (forty four) is a common noun. A proper noun is the name of a person, a place, a thing, or a title. Some examples of 44 as a proper noun:
Forty Four, AR (population 155)
Forty Four Cafe, 44 Northeast 1st Street, Miami, FL
44 Stone Public House, 3910 Peachtree Drive, Colombia, MO
