Both the singular and plural are the same, so it is 'have'. E.g. 'I have' (singular), 'we have', 'they have' (plural).
When used as a noun, the plural of have (meaning a person who has possessions; a trick or swindle (old slang)) is haves. Its most common usage is in the expression 'haves and have-nots'
The plural form for the noun arch is arches.
The noun 'fungi' is the plural form of the noun 'fungus'.
The plural form of inadvertence is inadvertences.
The plural form of itch is itches.
The plural of verandah is verandahs.
There is no plural form. Do and Do not are verbs
The plural form of him, her, or it is them. (objective pronouns)
Bridges is the plural form of bridge.
The plural form of "is" is "are."
The plural form of "I" is "we."
The plural form of "was" is "were."
"Groceries" is the plural form of "grocery."
"Beliefs" does not have a plural form, as it is already plural. Beliefs is the plural form of belief.
The plural form of mouth is mouths. The plural form of month is months. The Mounth is a range of hills in Scotland and does not have a plural form.
The plural form of the demonstrative pronoun 'that' is those.
The plural form is metres.
Trenches is the plural form.