This question already has an answer here:
Which expression is more idiomatic or correct:
- We are American
- We are Americans
Should I use American in the singular or in the plural form? And why?
You can virtually force one version or the other by context:
Are you Americans or Canadians? ...
Are you Canadian? ...
I don't think RyeɃreḁd's answer will really stand much scrutiny, though doubtless there are factors nudging one's choice one way or the other. And it is a choice:
'American' here is a (proper) adjective; compare 'We are cold'. It has no 'plural form'.
'Americans' and 'American' here are proper nouns; compare 'We are doctors'.
An Ngram shows that both constructions are commonly used. But you wouldn't get many Brits, standing in a defined group of people or otherwise, volunteering 'We are Britons' / 'Englishmen'. 'We are British' / 'English' is the far more usual way of putting this (as further Ngrams illustrate).
You would say "We are Americans" if you are standing in a defined group of people (can be counted) and talking about your group as individuals.
You would say "We are American" if talking about an abstract uncountable population or a group of people that will stay grouped.