English grammar rule A plural
The indefinite articles A or an modify singular nouns only. They do not modify plural nouns. However, when some numbers (hundred, dozen) modify nouns, indefinite articles also can replace one (one dozen, one hundred). Some informal expressions also use the construction many a, and in this case the noun must be singular (many a time).
Examples of 'A plural' rule
Incorrect We have dozen eggs. Correct We have a dozen eggs. Incorrect Jimmy played a games with friends. Correct Jimmy played games with friends. Correct Jimmy played a game with friends. Incorrect Many a things have deteriorated lately. Correct Many things have deteriorated lately. Correct Many a thing has deteriorated lately.