The correct statement is: An adjective has to agree both in number and gender with the noun it is describing.
Why should adjectives be in agreement with number and gender?
In many languages, particularly those with gendered nouns like Spanish, French, and Italian, adjectives change form depending on both the gender (masculine or feminine) and the number (singular or plural) of the noun they modify.
Adjectives are words that describe nouns, and they must match the noun in both number (singular or plural) and gender (masculine or feminine). This is a grammatical rule in many languages, including Spanish, French, and German.