Can CBOW model only accept fixed number of words? I have a question about CBOW prediction. Suppose my job is to use 3 surrounding words w(t-3), w(t-2), w(t-1)as input to predict one target word w(t). Once the model is trained and I want to predict a missing word after a sentence. Does this model only work for a sentence with four words which the first three are known and the last is unknown? If I have a sentence in 10 words. The first nine words are known, can I use 9 words as input to predict the last missing word in that sentence?
That's a good question. The answer is yes and no. 

No because the input layer of the CBOW model expects a fixed number of words. So you'll either always input 3 words or 9 words. 

Yes because you can however set the sequence length as 9 words and provide just 3 words as context while the remaining 6 words can just be zero vectors. Remember in a CBOW each word is represented by a vector.