Optimus is right. 국민"들" is plural. (If we have to differentiate: 국민=national, 시민=citizen) However, in Korean, the grammar for singular/plural is not very strict. If you want to emphasize plurality, you can add the suffix '-s,' but there is no strong restriction. For example, 거리에 사람이 많아 = 거리에 사람들이 많아. both sentences are grammatically okay.

국민 might mean a citizen, I'm not sure but whatever it means it is singular and 국민들 is plural