There's a class of nouns (mainly, but not necessarily, formed with suffixes of Greek origin): barista, autista; idiota, pilota; collega and so on, ending in a, that don't vary by gender in the singular but do in the plural. Barista is one of them: il/la barista, i baristi, le bariste
There's another class of nouns, ending in e: cantante, presidente; inglese, albanese; imbecille, debole, etc, that's don't vary by gender but do vary by number: il/la cantante, i/le cantanti.
Finally, there's a class of nouns (mostly borrowed from other languages or compound, usually ending in vowels other than o), that don't vary by number or gender: inca, aymara; madrelingua, baciapile and so on.