In a conversation someone claimed that certain sects allow for a greater limit on polygamy such as 9 or 18 women based on their understanding of the Qur'an. Is this true? How do they argue their stance?
This stance is attributed to some of the Khawarij and some Shi'as although I think that it is not accepted by the vast majority of the latter. And it is certainly not accepted by the Ahl al-Sunnah. In short it is based on the possible interpretation of 4:3 as follows:
انكحوا ما طاب لكم من النساء مثنى وثلاث ورباع
marry the women you like, two or three or four
marry the women you like, two and three and four
marry the women you like: two, two and three, three and four, four
The conjunction و can mean 'and' (addition) as in the following verse:
فصيام ثلاثة أيام في الحج وسبعة إذا رجعتم تلك عشرة كاملة
Fast of three days during Hajj and of seven when you have returned. Those are ten complete [days].
Or it can be used as a synonym for 'or' as in the following verse:
تقوموا لله مثنى وفرادى
Stand for Allah in pairs
Hence a group interpreted 4:3 as 'marry two and three and four' which is 2+3+4 = 9
Others further argued that the use of the expression مثنى وثلاث ورباع indicates pairs, i.e. pairs of two, three and four. Hence they claimed that it means (2 x 2) + (2 x 3) + (2 x 4) = 4+6+8 = 18
Some even claimed that there is no limit.
Other than the verse, they relied upon the example of the Prophet Muhammad ﷺ - as he had nine wives at the time of death (Nasai, Bukhari). And we are told to follow his example.
The majority however do not accept these interpretations because:
It is contrary to the interpretation of the verse from the Sunnah. It is recorded that before accepting Islam Naufal bin Mu’awiya had five wives, Ghailan bin Salamah had ten wives and Harith ibn Qays has eight wives. The Prophet ﷺ told each of them to keep any four wives and to divorce the rest. This proves that the limit is four.
The Quran is eloquent. It is argued that if nine (تسع) or eighteen (ثمانية عشر) were meant then using the names of those numbers might have been more appropriate.
It is opposed to consensus of the reputable scholars.
None of the Sahaba or Tabieen are known to have married more than four women.
In this issue the law for the Prophet Muhammad ﷺ is different from the law for the rest of the Ummah. The Prophet ﷺ is excluded from 4:3 as he was specifically permitted to marry as many women as he wished (Quran 33:50-51). And also because the limit in 4:3 is imposed to reduce the chance of injustice to the wives, whereas the Prophet ﷺ can not be unjust.
وقال بعض الخوارج: الآية تدل على جواز تسع: مثنى باثنين، وثلاث بثلاث، ورباع بأربع، ومجموع ذلك تسع، وبعض منهم تدل على ثمانية عشر: مثنى اثنين اثنين، وثلاث ثلاثة ثلاثة، ورباع أربعة أربعة، ومجموع ذلك ما ذكر، وهذا خرق للإجماع
Also see Tafsir al-Qurtubi, Tafsir Ibn Kathir, Tafsir al-Baghawy