A plethora is an overabundance. The excess here is key; in traditional usage of the word, a plethora is too much of something. Today, however, the word is often used as a synonym of plenty or many, which imply abundance but not necessarily overabundance.
But decision science has shown that people faced with a plethora of choices are apt to make no decision at all. [Daily Beast]
The plethora of mixed motives for the west’s engagement with the Arab world make doing the right thing harder in the Middle East and North Africa. [The Guardian]
I spent a few hours Monday on the ministry’s website, and was nearly driven mad by the plethora of acronyms. [Globe and Mail]