It says korach rebelled with 250 nesiim (princes). how were there so many nesiim? Wasn’t there just one nasi of each tribe, so 12 in total.
The 250 people that Korach rebelled with, were not nesi’im, which means governors, or leaders of a tribe, as you write, there were only twelve tribes. Rather the torah says they were “nesiay eidah” meaning “leader of the assembly”. Rashi explains that they were heads of the courts, meaning judges and people of stature. Rashi also says that most of them were in fact from the tribe of Reuvain.