Hello Wolfiya,

Hostel allotment at BHU UG programs is done strictly on the basis of merit. However, you may have an added advantage if you belong to the SC/ST quotas, who get about ~22% reservation of seats. There is NO reservation for OBC candidates, though. 

The exact marks requires varies from time to time depending on particular factors, and you should check this link: http://www.bhu.ac.in/accommodation/ for the 2018-19 session.

It also provides additional info like the hostel fees, and accomodation arrangement in case you have a spouse/family.