In the Keras Tokenizer class, what exactly does word_index signify?
Are the words all words in texts, or are they maxed at SOME_NUMBER?


Yes, it will be maxed at the most common SOME_NUMBER-1 words.


  And are the dict values for word_index the frequency of each word, or just the order of the word?


It is just the index of the word in the dictionary.

You can read more here in the documentation.