No way to query for all terms. With get_terms() from WordPress you can pass the number arg as 0, or it defaults to 0, to get all the terms with the associated taxonomy. The terms controller uses per_page to fill out the number arg. The default for per_page is 10, and you can't pass 0 to this argument (or any negative numbers, because it sanitizes the value with absint). This effectively makes it impossible to query for all terms in a taxonomy.