I'm pretty sure the correct translation is "Amerikaner", as with most nationalities.
"Amerikaner" is singular. "Amerikanere" is plural. When the singular ends in an R, you usually have to add an E to get the plural. A lot of nationalities end in R in Norwegian (mexicaner - mexicanere, canadier - canadiere, japaner -japanere, etc). We also don't capitalize nationalities like you do in English.