All prisoners are allowed some visitors by law so yes even maximum security prisoners and even those sentenced to life in prison without parole can have visitors.
The warden of the prison can restrict the number of visitors at a time and the length of the visits as they deem necessary.
Most prisons only allow a prisoner to have a maximum of 5 visitors per inmate and restrict the visits to around 1 or 2 hours but if the person or people visiting the inmate lives farther away sometimes special arrangements can be made to allow for a little longer visit.