Normally PCB which only one conductor is single layer PCB. Common conductor are copper , AgPd (Silver Palladium) AuPd (Gold Palladium ), Al(aluminum). Normally, for FR4 PCB, metal core PCB and Flexible PCB, the conductor are copper. For ceramic PCB, the conductor can be copper, AgPd and Aupd. Here some common structures for your reference.
single side FR4 PCB
single side ceramic PCB
Of course, there are some special circumstances, even only one conductor, but they are not single layer PCB, they are named Dual Access PCB.
For example:
dual access FR4 PCB
