Counting words of a text file can be accomplished with a command wc, which stands for word count. Please note that these instructions only apply to text files, not binary files.
If your file is named as myfile.txt, type following command in the terminal:
$ wc -w myfile.txt
w stands for words.
The output is the number of words in the file.
If you want to count number of word occurences you need more sophisticated method. Read this: Count Word Occurences in File in Linux