what will happen if i score 13 out of 122 , but failed to get 7.5 %in one subject of paper 2 ? ST category

There is no individual cut off for each subject in each paper. Subject cut off is combined of both the paper marks.So if you score more than 10% of total aggregates in any subject then your cut off that Subject is achieved you don't have to score individually 10% in each paper.