UPSC prelims has two papers: GS Paper-I and CSAT (Paper-II).
GS Paper-I decides the prelims cut-off, which UPSC announces after the final result.
CSAT is qualifying in nature — you must score at least 33 percent in it.
There is no fixed qualifying mark for GS Paper-I; the cut-off varies each year with paper difficulty and vacancies.
UPSC releases the official cut-off marks for prelims along with the final result.