You need to choose the amount of dibasic and monobasic to get the pH you want. You can look at the table below to help you:

https://www.semanticscholar.org/paper/Dynamic-approach-to-predict-pH-profiles-of-relevant-Ganesh-Soumen/bba13588d5195ec35f761ce23e44cd3f12955cf7/figure/2

As suggested by Nicolas, it is best to mix two components to achieve the desired pH, requiring only minor adjustments if needed. It requires less effort. You can use a single component, but this would require pH adjustment with a strong acid or base. The final salt strength should keep the same as the sum of both components.

Thank you for your help David Liang and Nicolas Poirier .

I didn't realise we had to use both components :)

It is preferable to use both chemicals for phosphate buffer preparation, as mentioned earlier. However, using just one will still work. If you use only Na₂HPO₄, you'll need to add more acid to reduce the pH.

Thank you David Liang . I’ll try it out with the HCl method and see how I go. The stakes are low as these are practice cells 😅