Writefull uses large language databases to search for the frequency of chunks of text. The databases used are Google Books, Google Scholar, Google News, and Google Web. All you need to do is select a chunk of text (in whatever writing tool you use), activate the Writefull popover, and request the information you need. Check out the tutorial for more information.
The way Google Books organises its data is in ‘groups’ of tokens, ranging from 1 to 5 tokens each. So when searching the database for a chunk, you cannot search for more than 5 tokens at once. Note that tokens also include symbols such as the apostrophe (‘) and hyphen (-); so don’t count words only! A selection of ‘this isn’t your well-being’ contains 5 words, but 8 tokens. This is a limitation of Google Books, and is exactly why we also provide Web searches.