Ja-Singular: Yes, I should. Yes, you should. Yes, he/she/it should.
Ja-Plural: Yes, we should. Yes, you should. Yes, they should.

Nein-Singular: No, I shouldn´t. No, you shouldn´t. No, he/she/it shouldn´t.
Nein-Plural: No, we shouldn´t. No, you shouldn´t. No, they shouldn´t.