* Dictionaries are not exhaustive: Dictionaries only include a portion of the words in the English language.
* New words are constantly being created: The English language is dynamic, with new words emerging all the time.
* Defining "word" is tricky: Do we count proper nouns, technical terms, slang, and other categories?
However, we can make some observations:
* "Them" as a word: The word "them" itself is common.
* "Them" as part of longer words: Many words contain the letters "them" within them, such as "anthem," "thermal," "thematic," and countless others.
To get a rough estimate, you could try searching a large online dictionary or corpus for words containing "them." But be aware that the result will be an approximation.