I should have made three cups of rice instead of two.
Two cups of rice wasn't enough. Three would have been better.
ーI should have made three cups of rice instead of two.
「2合じゃなく、3合のお米を炊けば良かった」
should have で「〜べきだった」
three cups of rice で「3合の米・お米3合」
ーTwo cups of rice wasn't enough. Three would have been better.
「お米2合じゃ足らなかった。3合の方が良かったな」
wasn't enough で「十分でなかった」=「足らなかった」
ご参考まで!