The answer will vary wildly per language and will depend on how new words are created in a language.
In an agglutinative language, words are created by joining words together. For instance, a chair you use at a desk in the office, an office chair, is one word in German, "ein Bürostuhl" (literally a "deskchair").
As you can see, that way new words are created easily and on the fly. That is why Icelandic has such a large dictionary, which is not even exhaustive — it can't be.
So the answer to your question will be it depends. Perhaps you should restrict your question to a specific language, as not all languages are as liberal in word formation.
Dutch: 652,000 words
The Dutch Scrabble association, the Scrabble Bond Nederland (SBNL) uses the Scrabble Word List (SWL) for words up to 9 letters and publishes separate lists for 10, 11, and 12 letters.
The SWL contains over 242,000 words, the supplemental lists contain about 125,000 words, about 141,000 words, and about 144,000 words respectively, for a grand total of 652,000 words of length 2 to 12.
English: 276,000 words
The World English-language Scrabble Players Association (WESPA) states in its rules
Global word source
The word source used in global Scrabble, known as Collins Scrabble Words or CSW (formerly Official Scrabble Words or OSW) is actually not a single dictionary, but an amalgam of three: Collins, Chambers and TWL, the current Northern American wordlist.
TWL (Tournament Word List) is itself drawn from a range of sources, mostly different editions of Webster's. Americans and Canadians play by CSW rules in international tournaments and TWL in 'domestic' North American tournaments. But they are increasingly introducing CSW grades in various tournaments, and even some solely CSW tournaments. WESPA hopes this North American isolationism will end one day, for the good of global Scrabble.
Collins states on its website:
(...) Collins Official Scrabble™ Words 2015 is the most comprehensive Scrabble™ wordlist ever, including World English from Australia, Canada, New Zealand, South Africa, UK, and USA brought together in a single list.
With over 276,000 permissible words, including inflected forms, this is a must for anyone playing Scrabble.
French: 393,670 words
The French Scrabble association, La Fédération Internationale de Scrabble Francophone (FISF), uses L'Officiel du Scrabble which is currently in its seventh version, containing 393,670 words from 2 to 15 letters.
The German Wikipedia article on Scrabble quotes a source about "120,000 words".
Scrabble Deutschland e.V. states the official list contains 179,998 words of 2 to 9 letters.
I'll add more sources and languages as I find them.