Phosphoric acid, #H_3PO_4#, is the parent acid. If it loses a proton, #H^+#, we conserve both mass and charge, and #H_2PO_4^-# results. And what is the conjugate base of this beasty? Again, conserve mass and charge, and #HPO_4^(2-)# results. You did not conserve mass and charge in your question; I agree that this is all too easy to do.
What is the conjugate base of biphosphate, #HPO_4^(2-)#? This species does not exist in water. And what is the conjugate base of bisulfate, #HSO_4^-# (and this one does exist in water)? Can you tell me the conjugate acid of #H_3PO_4#? Conserve mass, and conserve charge, and these are trivial questions.
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