Plenty refers to much more than one of something. It is already plural in a sense. The word can not be used to describe singularly.
the plural of route is routes. As in "there are plenty of routes to take".
It is singular.
The plural of fresco is frescoes. As in "Italy has plenty of frescoes to view".
The noun plenty can function as an indefinite pronoun when used to take the place of a specific number. As an indefinite pronoun, the word plenty can be plural or singular.Examples:Plenty are dissatisfied with the government's plan. (plural, a great number)We don't need oranges, we have plenty. (singular, a sufficient quantity)
That depends on the subject of the sentence. For example a first or second person subject, such as "I" or "you" would require "need." I need to drink plenty of water. Third person, with the exception of third person plural, uses "needs" John needs to drink plenty of water. Also, any time a group is referred to in the singular such as "this team" you use needs. However, if you were to say a proper name like "the cardinals" you would use need. In short it's: I/you/they/we need to drink plenty of water. John/he/she/ needs to drink plenty of water. "The basketball team" needs to drink plenty of water. "The Globetrotters" need to drink plenty of water.
The plural of activity is activities.Two example sentences are:There are plenty of activities to do at the club.The police are looking into the activities of the criminal gang.
The plural of 'this' is 'these' and the plural of 'that' is 'those'.
Isthmi is plural. There are actually two plural forms. I will list them from singular to plural. Isthmus - Singular Isthmi - Plural Isthmuses - Plural
The plural of rose is roses. The plural possessive is roses'.
The plural of 'bunch' is bunches.The plural of 'ant' is ants.The plural of 'batch' is batches.The plural of 'day' is days.The plural of 'chimney' is chimneys.The plural of 'tomato' is tomatoes.The plural of 'umbrella' is umbrellas.The plural of 'donkey' is donkeys.The plural of 'sky' is skies.The plural of 'foot' is feet.The plural of 'show' is shows.
The plural form of can is cans.
applied is does not have a plural but is apply it does have a plural.