"Many girls sleep."
Translation:Multae puellae dormiunt.
Adjectives must agree with the noun they modify in number, gender, and case. You can see a full chart of the adjectives forms at Wiktionary: https://en.wiktionary.org/wiki/multus#Declension
multi will either be masculine or neuter (depending on the noun it modifies) but never feminine so it cannot match with puellae (feminine noun). multae works here since it is feminine, plural (since it is 'girls'), and nominative (since the 'girls' are doing the action).