N is the number of turns in a single coil of the motor windings. The maximum value of flux is the flux that the iron core for the coil can carry without being saturated. N and Phi are selected by the motor designer.
Once the motor has been designed, everything that is of interest to the user of the motor is marked on a rating plate that is permanently attached to the motor. If the motor has been designed for a manufacturer to incorporate in another product, the designer provides documentation that is identified by the motor's identification number and only that ID number is marked on the motor. The design information may be published in catalogs, but it is often provided only to the original purchaser.
If the rating plate is available, the V/f is calculated by dividing the motor's rated operating voltage by its rated operating frequency. If the rating plate is not available, the rated voltage and frequency are often the same as the rated voltage and frequency of the product in which it was used. Sometimes it may be necessary to examine the product to determine if the voltage and frequency applied to the motor were internally derived inside the product.
Note that a constant value of V/f is strictly applicable only at the rated voltage and frequency. It is necessary to determine by observing the motor performance how much V/f adjustment may be necessary as the operating frequency is reduced and increased. Usually constant V/f is sufficient for frequency reductions to 10% of rated frequency or so. If the load torque declines as speed declines as with fan or centrifugal pump, constant V/f is sufficient for any speed reduction.