Are the following sentences correct grammatically?
1- The war had two hundred woundeds. (And not wounded soldiers)
2- There are two modals in that sentence. (And not modal verbs)
That is, can we add plural s to the adjectives?
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. ]
I'm not a native English speaker. But, what I help you best is introducing this link to you.
It says that "modal" can be a noun. Having searched on the Internet, yet, I haven't found any clue about "woundeds".
Have a nice English hunting. :)