Other contributors have said "How many words in dutch language?" is the same question as "How many words are there in the Dutch language?". If you believe that these are not asking the same thing and should be answered differently, click here.
According to the "Dikke van Dale", the most respected Dutch dictionary, there are approximately 240.000 words (translated from http://www.ikhebeenvraag.be/vraag/7996).
The director of Dutch institute of Lexicality estimates more then 5 million words are ever used in the Netherlands. The language Databank, a computer that registers all words and word-forms, registered 60 million words - including conjugations of verbs. (translated from http://taalunieversum.org/taal/vragen/antwoord/9/).
However the amount of combinations and new words is infinite, and therefore it is difficult (if not impossible) to give a correct estimation.