This question is a bit tricky, because it depends on the application.
The BTS7960 is a brushed DC motor driver with PWM speed control, so I presume you have a permanent magnet brushed DC motor.
Compared to using 24 V, running the motor on 12 V will:-
Draw half the stall current and half the initial startup current.
Have half the torque output at stall.
Spin half as fast at the same PWM ratio.
Draw the same current for the same torque output.
Draw twice as much supply current at the same speed with the same load.
For example if it needs 50% PWM to get the required speed on 24 V then it will need 100% PWM on 12 V, drawing twice the power supply current for the same power output (torque*rpm). This is necessary because power = voltage * current, so at 12 V the current must be doubled to get the same power input.
If the motor is geared and you change the gear ratio to get the same speed then it may draw twice the current or more, depending on where on its efficiency curve it is operating.
If the motor is driving a load with very high startup torque (eg. a piston pump or compressor) then it may not have enough torque to start up - and so will stay stalled drawing much more current than it would at 24 V.