Learn Italian: Voglio provare vari mascara. - I want to try various mascaras.

mascara is an imported foreign word. In Italian, most foreign loanwords are invariable, meaning they do not change their spelling in the plural. We know it is plural here only because the plural adjective vari is modifying it.

Why is it vari and not varie?

vari is the masculine plural form of the indefinite adjective vario. Even though it ends in 'a', mascara is a masculine noun in Italian, so the adjective must agree with it. If you were talking about a feminine plural noun like creme (creams), you would use varie.

Could I use diversi or parecchi instead of vari in this sentence?

Yes, they are grammatically correct but carry slightly different nuances. vari emphasizes the variety, meaning you want to try different brands or formulas. diversi also means several or different, while parecchi emphasizes quantity, meaning you want to try quite a few mascaras.

Why is provare left in the infinitive form?

Because it directly follows voglio (I want). Just like in English, when a conjugated modal verb like volere (to want), potere (to be able to), or dovere (to have to) is followed by another action, that second verb stays in the infinitive (dictionary) form.