Starting in R2014a, variant subsystems can have inports and outports that differ in number from the inports and outports in the parent Variant Subsystem. This is not supported in earlier releases. However, it's possible to model these systems by attaching some inputs and outputs to Terminator and Ground blocks.
To do this, create the child subsystems. Each should have N inputs and M outputs (some will be "dummy" inputs or outputs). Then, for each subsystem, connect the dummy inputs to Terminator blocks and dummy outputs to Ground blocks.
