According to the Global Detention Project there are 51 operating detention centers in Libya. According to the IOM there are 24 centers (May 2019) (Infomigrants, 2019/07/11). The same article states that 15 of the centers are to be closed and that there are 5,800 migrants being held according to the UNHCR. ReliefWeb writes in Oct 17, 2019 that they are not freeing the internees but just moving them to even more crowded places.
The abuses of the refugees are well documentet. Check the links for more on this.