int virtualChildCount

The overall number of children this viewport could potentially display.

If null, the viewport might display an unbounded number of children.

Source 

int get virtualChildCount => _virtualChildCount;
void virtualChildCount=(int value)

Source 

set virtualChildCount(int value) {
  if (_virtualChildCount == value)
    return;
  _virtualChildCount = value;
  markNeedsLayout();
}