What is the plural form for do and don't?

Those are the 'plural forms', "do" and "don't"; for example when you say, "they do" or "they don't".

There are some grammatical contexts, however, in which "do" and "don't" will represent the 'singular form', such as when you are talking in first-person narrative about yourself, as in "I do" or "I don't". In that particular case the context is not plural for these same Words.

(As a side note, the usual 'singular forms', outside of first-person narrative, would be "does" and "doesn't", as in "she does" or "she doesn't").