The noun proportion can be countable or uncountable. In more general, commonly used, contexts, the plural form will also be proportion. However, in more specific contexts, the plural form can also be proportions e.g. in reference to various types of proportions or a collection of proportions.

Is proportions singular or plural?

Proportion is singular, so have is correct.

Is the word proportion countable or uncountable?

[uncountable, countable, usually plural] the correct relationship in size, degree, importance, etc. between one thing and another or between the parts of a whole You haven't drawn the figures in the foreground in proportion. The head is out of proportion with the body.