When using Plethora in a sentence is it plethora or are Like there isare of ideas etc?

"Plethora" is a singular noun. It means a large or excessive amount of something. So, you would use "a plethora" in a sentence, not "are" or "is."

Here are some examples:

* There is a plethora of ideas for our next project.

* She has a plethora of hobbies.

* The market offers a plethora of choices.