How to design a component which has N number of Avalon ST Sink ports in SOPC Builder

It's probably easiest to do this by coding up the maximum number of ST interfaces your component will support, feed it through component editor, then modify the .tcl file to add an elaboration callback which will stub out interfaces based on the parametrization. In the SOPC Builder handbook there is a chapter on the .tcl API you use to do this.