According to my knowledge, back in the Middle Ages, as Muslims rose in rank they were expected to have more wives with, kings being expected to have a total of four wives. Was there a similar case with Ahl-i-Batin considering they were based on Muslim traditions?
Short version: unless there's a WoD sourcebook that specifically addresses this point, there's room for you/your storyteller to play this however you would prefer.
back in the Middle Ages, as Muslims rose in rank they were expected to have more wives with, kings being expected to have a total of four wives
Depending on what cutoff points you accept, the Middle Ages spanned about a thousand years, overlapping with Islam for about eight hundred years. The medieval Islamic world extended from Africa to China and Indonesia. I would be wary of any source that makes statements about "Muslims in the Middle Ages" without qualifying when, where, and who.
Islamic attitudes towards polygyny vary greatly. While the Quran does set out rules for polygyny, that's not to say it encourages it. One common reading of those passages is "at a time when there was a shortage of men due to losses in battle, polygyny was permitted as a way to provide support to widows, but those circumstances no longer apply". The Quran stipulates that a man is only allowed multiple wives if he can treat all of them justly, and then questions whether anybody can really do so.
(Noting also that polygyny was common in pre-Islamic Arabic culture, so these passages actually represent a considerable curtailment of polygyny, not an expansion of it.)
For these reasons, many Muslims consider polygyny to be deprecated, and in many modern-day Muslim countries it's illegal. Even in the Middle Ages, it was by no means universal for kings to have four wives; to the best of my knowledge only one wife is recorded for Saladin.
As an obscure religious sect, it'd be unusual if the Ahl-i-Batin didn't have some peculiarities in their interpretation of their religion, and World of Darkness has never been known for its close adherence to RL history. It would be quite plausible to go either way on this.