What is the correct value of INS_ACC_BODYFIX?

The default value is 2 (2nd IMU) but what happens with FCs that only have one IMU?

Thanks in advance.

Actually in most cases it is irrelevant, since all IMU’s are body fixed in a common flight controller.
In case you have external IMU’s and some of them are not aligned with the vehicle body, then acc calibration will select the INS_ACC_BODYFIX imu for reference. In systems with one imu you can set it to 1.
Nota bene : board_orientation will be applied to the body fixed imu’s data, so it does not depend on board orientation.

If no IMU exists with the set number then it fall back to available imu’s and assume that it is body aligned.