Are field marshals, captains general and generals of the army appointed only in time of war, when the number of troops in the field is so large that it is necessary to have a top general in charge of all generals in service?
Depends on the date.
In the British army the rank of Field Marshal used to depend, in theory, on the choice of the Sovereign regardless of service.
I believe towards the end of the Victorian period the number of Field Marshals was around six.
Today Prince Charles and the Duke of Kent hold the rank [honorary] along with around four others who have earned it still above ground,
Here is another picture of the newly promoted Abdel Fattah Al-Sisi in the less formal everyday (winter) uniform. Largely the same as the formal portrait uniform, but lacking the gold border around the epaulets.