Buffers with pH from 5.8 to 8.0 can be prepared by mixing NaH2PO4 and Na2HPO4 solutions in various proportions, according to tables that can be found online.

You can use acid or base to adjust the pH outside of that range, but the solutions will not necessarily be effective as a buffer at all pHs, because buffering capacity is maximal at a pH close to the pKa of the buffering agent. For the NaH2PO4 and Na2HPO4-based buffers, the relevant pKa for the H+ + HPO42- H2PO4- equilibrium is about 7.2.

The pKa for the HPO4- PO43- + H+ equilibrium is about 12.4, so you could titrate Na2HPO4 with NaOH to make buffers in the range of about 11.4 to 13.4.

The pKa for the H2PO4- + H+ H3PO4 equilibrium is about 2.2, so you could titrate phosphoric acid with NaOH to make buffers with pHs from the pH of phosphoric acid (~1.5 for a 0.1 M solution), up to about 3.2.

So, certain pH ranges are not accessible for buffering with phosphate, and other buffers should be used. Here is a buffer reference site:

https://www.sigmaaldrich.com/US/en/technical-documents/protocol/protein-biology/protein-concentration-and-buffer-exchange/buffer-reference-center#phosphate