As the name implied, 2 layer flex circuits stackup means there are two copper trace in it.
Standard layer stack up for 2 layer flex circuits from Best Technology, there are 8 types for adhesive and 5 types for adhesiveless.
When the circuitry is too complex to rout or the copper foil is needed for ground shielding,
then the multi-layer board is required. Further details of 4 layer flex circuits stack up of Best Technology as below, adhesive and adhesiveless
both are listed.