What is correct Need to drink plenty of water or needs to drink plenty of water?

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.