Can adjectives get plural s?

No, adjectives in English do not take the plural ending.

Having said that, word categories are quite fluid in English, and some adjectives may be used as nouns, in which case (if they are count nouns) they will pluralise like any other noun.

"Modal" is an example, though only in technical use (linguistics and programming). General examples are "blonde" and "characteristic".

I don't know of any rules for determining which adjectives are used as nouns in this way: "wounded" is not so used, in my experience.

[There is another way in which adjectives get used as nouns, representing the mass of things or (usually) people to which the adjective applies: always with a determiner such as 'the' and never with plural ending, so it doesn't directly relate your question: "The poor, the old, the infirm". In that context "The wounded were many" is fine. ]