They want to become a teacher/teachers.

Hi Teacher:

I have a sentence which I am not sure which subject complement to be used.

They want to become a teacher/teachers.

Thank you in advance for your help.

I would use the plural.

Hi Teacher Tdol

You wrote that you would use plural noun.
Would you mind telling me what kind of grammartical mistake it is if we choose the wrong complement?

Thank Teacher.

Concord - Glossary Definition - UsingEnglish.com
Agreement - Glossary Definition - UsingEnglish.com