Eight rhymes generally require two couplets in the middle to fight.
There are also some special formats, such as 13 couplets, but the couplets are wrong and called "stealing incense". For example, there are only six sentences in Xiaofa, and there is only one antithesis in the middle. Another example is the arrangement of more than eight sentences. Except for the beginning and end of couplets, all other couplets should be opposed.