Some sources say that italian plurals come from the nominative case, so "italiano" has the plural "italiani", and "italiana" has the plural "italiane".
However some sources say that the words in italian come from accusative, just like in other romance languages. So "carro" (from latin carrum) has the plural "carri" (from latin carros > carroi > carri), and "rosa" (from latin rosam) has the plural "rose" (from latin rosas > rosai > rose). This sound change can justify why the plural of "amica" is "amiche" and not "*amice" (amicas > amicai > amiche, preserving the [k]). However it could be a hypercorrection considering that the singular keeps the [k] sound. Another word that may prove this accusative claim is the word "noi", from latin "nos", it's the same sound change. And the last point is that if a masculine word came from the nominative case, it would come from a "-us" ending, not a "-um" ending, so "carro" would come from "carrus", so it would be something like "*carros", maybe "*carroi" (> "carri"). Yet, nothing forbids, I believe, the fact that only the plural may have come from nominative, while the singular may have come from accusative. So, what do scholars say? Which theory is probably right?