asalmu alaykum.1.How many wives did Al Hasan Ibn ali(Ra) have??2.The most detestable of lawful things near Allah(swt) is talaq(ibn mazah)-is this authentic narration??

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger. {C}

We have not come across a specific number of wives that Al-Hassan ibn Ali had married.

Some scholars stated that he married about seventy wives but this number may be exaggerated. However, if it is proven that this number is true, then there is no objection as long as he abided by the Sharee'ah.

Ath-Thahabi said in his book Siyar a’laam An-Nubalaa’:

"He was always marrying and divorcing; he married about seventy wives, and he rarely had less than four co-wives. Ja’far As-Saadiq narrated that Ali (the father of Al-Hasan) said: “O people of Koofa, do not marry (your daughters off to) Al-Hassan, as he always divorces his wives.” A man said: “By Allah, we will marry (our daughters off to) him, he may keep whomever wife he is pleased with, and divorce whomever he wishes to divorce." [End of quote]

The Hadeeth that you asked about was reported by Abu Daawood, Al-Haakim and others, and it was classified as inauthentic by many scholars, including Shaykh Al-Albaani.

The jurists indicated when speaking about divorce that the five rulings apply to it: meaning that it may be obligatory, desirable, permissible, disliked, or forbidden, depending on the circumstances.

Allah knows best.