You could write all physics in a single simple equation. deltaW=0 Where deltaW is deviation of the universe from the relevant math.

Writing Maxwell's as 1 equation or 4 or more is just esthetic choice where you decide what to accentuate.

20 might be too much because three dimensions are not really different from each other so the notation that maps over them wholesale is probably a good idea.

4 equations seem perfect if you want to differentiate between classical effects of the electric field and relativistic effects (magnetism).

I don't know if single equation really shows that they really have the same source and the relativity is involved or is it just a matrix mashup of the 4 separate equations that doesn't really provide any insights.