As it is planned to keep the volume n constant over the next twelve months, in the mathematical model the common approach is to put n = 1. You can do this, because nothing depends on the volume and the end result will be the same. Now the total cost will be simpler:

f\left(\omega\right) = p_A \omega+ p_B \left(1 - \omega\right) \tag{1}

This is done because company need to buy product on monthly basis. If it was budget for 12 months combined, then n wouldn’t be set to 1.