Is have the plural of has?

Answer 1

Have can be both singular and plural, but has can only be singular. So you are partly right.

Have is used with the first and second persons singular and with all persons plural and plural noun subjects:

I/You/We/They have a large fat dog. The boys have a large fat dog.

Has is used only with the third person singular and singular noun subjects:

He/She has a small dirty dog. The doctorhas a small dirty dog

Answer 2

No, "have" is the plural of "has." "Has" is the third person singular form of the verb "have," while "have" is used for first person singular, second person singular, and all plural subjects.