How slowly can a brushless DC motor spin?
A brushless DC motor can be operated as a stepper motor. Operating stepwise, it can operate as slowly as you want. Smooth operation would be more difficult. Determining that would probably require a simulation with complete characterization of the motor, controller and load.
Of course, the motor cooling would also need to be considered.
If a "typical" brushless motor and controller are defined as those designed to drive a fan or a drone propellor, operation below 1/3 of rated speed is probably not advisable except for as short a time as possible when starting and stopping. Very little torque required at low speed with that kind of application. With the right controller design, these motors are probably capable of producing the same torque at standstill as at rated speed, but thermal limitations would severely limit low speed operating time.
Brushless motors and controllers that are designed to drive electric bicycles, motorcycles and cars can be expected to be much more capable of low-speed operation. With the best controller design, they can probably operate at very low speeds. The thermal limitations would not be as severe as with fan and propellor motors, but still an important limitation.
The number of poles is not necessarily important. Quantified speed, torque and other requirements are more important than anything else. Load characteristics are important. Duty cycle is important. The ratio of the slowest to the highest required speed may be important.
The more poles a brushless motor has, the smoother it will spin at low RPM with a trapezoid/square wave drive. But if you just use a sinusoidal drive then it doesn't really matter. Of course you still need an appropriate rotor feedback mechanism for the RPM you are spinning at. Some are ineffective at low speeds. So it's more about the drive than it is about the motor.
The brushless motor can spin at any low speed. Maintaining low speed smooth may require some algorithms, feedback hardware and stuff, but is totally doable. For example with incremental encoder you can use 1/T method (measure time between pulses rather than count them) to sense the speed. With high resolution encoders even that may be not necessary.