What decides the position of Hall Sensor in BLDC motor
The position depends on the design of the motor, but the position of the sensor is not magic.

The following image shows the most simple BLDC motor with just one pair of coils and a rotor with two poles, at the moment when polarity is changed:



From left to right:


The e-magnets still attract the the poles the permament magnet of the rotor and create torque
The polarity of the e-magnets changes
The e-magnets repel the poles of the stator facing them and attract the poles on the opposite side, creating torque


The hall sensors of these BLDC motors often already contain a comparator giving a high or low level, indicating if they see a N or S pole.

In the example above, such a sensor would be placed where the blue triangle is. The electronics behind would make it so that the polarity of the e-magnet on the right is always the same as what the sensor sees. 

You'll find this principle in that fans used in computers, though there are two pairs of coils and the rotor has four poles:


(Source)

Note that the sensor is placed so that it sees the transition when the poles of the rotor are exactly aligned with the coils!

Another type of motor uses a rotor with two poles and three pairs of coils:


(Source)

In the most basic mode of operation, each pair of coils changes its polarity when the rotor poles are aligned to it, i.e. pair A1/A2 is just about to change in the picture. (In reality, it's more complex, see the source). So you could use three hall sensors in there, each responsible for one pair of coils.

So, now it depends on the design of the motor - how many coil pairs are used, and how many poled does the rotor have. With some logic or
some intelligence of the controller, it is also possible to spare some sensors.