One of the first tests for infringement is "substantial similarity"(see [1] for example). If you claim copyright infringement on the basis of any small number of characters your case will be tossed by the judge. There isn't a specific number of characters that need to be copied before it becomes "substantial" because the court gets to rule on that based on the context etc.

[1]https://www.legalzoom.com/articles/copyright-infringement-an...