There are some languages which do not have characters at all, because the people who speak them have not developed writing. Remember there are several thousand languages spoken on earth, and Unicode is a way of recording their alphabets. The languages preceded Unicode, so what you mean is how many languages Unicode has managed to record with characters < 0xffff. It is probably easier to find which languages use characters ≥ 0x010000. Go to the
Unicode website, open the charts for the different languages, and see which numbers they support. For example, Ethiopic comes up as U1200 (< 0xffff) and CJK Extension B comes out as U20000 (> 0x010000). CJK means Chinese Japanese and Korean.
You might find lists of characters somewhere else in the Unicode website, if you look for its history. That is all I can think of.
There are some languages which share character sets; for example all Western European languages use similar letters, and can probably be fitted into U0000…U0100. That would include Romany, English, Lallans, Gaelic, Cornish, Welsh, Latin, Huguenot and Yiddish. Not Hebrew or Chinese, which are Asian languages. Those are all languages which have been spoken in Great Britain for several hundred years, so you can see what a big list you would have. In some areas, eg New Guinea, there is a different language in each village, and people cannot talk to those who live ten miles from them.