To create a SyntheticSpectrumFromArray with a variable number of partials, you can set Amplitudes to:
{!Amp copies: 1024}
and NbrPartials to the desired number of partials.
This is simple enough that creating a class would not make it any simpler for people to set and change the number of partials. Only one parameter field has to be changed in order to change the number of partials.
By using copies:, you will get numbered faders starting with !Amp000. If you prefer to start from 1, you could use:
{1 to: 1024 collect: [ :i | !Amp suffix4: i]}
Don't worry, you won't create any extra faders because SyntheticSpectrumFromArray will create only NbrPartials anplitudes and frequencies.