The best course is to provide us with the document or other material to be translated and let us do a word count using our tools. If this is not possible, there are other ways to get an approximate count. Some programs, like MSWord, will provide a word count for you. There are also free tools available online such as Textomate that will count the words in many common types of documents. (You should be aware that the number you get will probably not match our final word count exactly since we use computer aided translation (CAT) tools for our counts and they may not use the same method of counting.)
If the material to be translated is still not ready, you can use a similar document or block of text to extrapolate an estimate of the amount of words that the document will have.