The promise of 72 virgins does not come from the Quran, it comes from a hadith of the Prophet Muhammad.

According to the web site http://eshaykh.com/hadith/martyr-and-72-virigins/

Hadith means “saying.” Hadiths are the second source of the Shari`ah, the first being the Qur’an. The Qur’an is all-binding and unquestionable as a source for Muslims, but hadiths need verification and authentication, hence the need to specify, when one of them is cited, that it is “authentic.” The nine most authentic compilations of hadith in Islam are Sahih al-Bukhari, Sahih Muslim, Muwatta’ Malik, Sunan al-Nasa’i, Sunan al-Tirmidhi, Sunan Abi Dawud, Sunan Ibn Majah, Musnad Ahmad, and Sunan al-Darimi.

The hadith in question is found in Sunan al-Tirmidhi and Musnad Ahmad and states:

“Truly the martyr is given by Allah Almighty six things:

1. Forgiveness of sins from the first drop of his blood to be shed;
2. Protection from the torment of the grave;
3. Safety from the Greatest Fear [i.e. the Day of Judgment];
4. Receiving the crown of veneration, a single sapphire of which is worth more than the whole world and its contents;
5. Marriage to seventy-two of the wide-eyed virgins;
6. Intercession on behalf of seventy of his relatives.”

The number “seventy” in Arabic denotes “a great number.” The reason for the wives being 72 is 70+2, seventy (i.e. many) of the Houris of Paradise and two of the women of this world, and Allah knows best.