Each ml of this solution contains 1 mmol of potassium ions, 1 mmol of phosphate ions, and 2 mmol of hydrogen ions.
Lets dwell on that for a moment.
The compound is composed of K+, 2H+ and PO43-. In fact it very closely resembles phosphoric acid (H3PO4) except one of the hydrogen atoms has been substituted with potassium.
Some textbooks (and specifically the MIMS PI for the DBL potassium dihydrogen phosphate) report authoritatively that at physiological pH the most numerous form of phosphate is the divalent PO42- form, outnumbering the monovalent form 4 to 1.
In other textbooks the reverse is stated.
One assumes that the monovalent form ought to be the dominant in the realm of extracellular fluid, given that the pKa for the monovalent form is roughly 7.5 and the pKa for the divalent form is 6.2.
The region in which phosphoric acids are in equilibrium with their conjugate base are pKa +/-2.