Accumulators are so 20th century.
But how many ?
Usually, a good rule of thumb is to use as many as your register width.
8-bits => 8 registers.
16-bits => 16 registers.
You see the pattern.
As to why this pattern exists, it's just a heuristic. Depending on the application and other constraints, this can change but these values are a good average for scalar processors.