The voltage changes depending on the number of batteries and the way you mount them together:
12 V : most of toys fitted with a motor;
24 V : some projects from schools;
36 V : choose taken by one manufacturer. This allow to only use 3 batteries, which gives a lower weight and reduce the cost of the set of batteries;
48 V : most go-karts;
60 V;
72 V : go-karts entering the FIA alternative energies championship;
84 V;
96 V;
108 V;
120 V;
132 V;
144 V : some dragsters used in USA;
etc ....