instanceCountForSelectedSymbol.js

var symbols = context.selection,
	symbolLoop = symbols.objectEnumerator(),
	symbol;

while (symbol = symbolLoop.nextObject()) {
	var pages = context.document.pages(),
		pagesLoop = pages.objectEnumerator(),
		page,
		master,
		count = 0;

	switch (symbol.className().trim()) {
		case "MSSymbolInstance" :
			master = symbol.symbolMaster();
			break;
		case "MSSymbolMaster" :
			master = symbol;
			break;
	}

	while (page = pagesLoop.nextObject()) {
		var predicate = NSPredicate.predicateWithFormat("className == 'MSSymbolInstance' && symbolMaster == %@",master),
			instances = page.children().filteredArrayUsingPredicate(predicate);
	
		if (instances) count = count + instances.count();
	}

	log(master.name() + " has " + count + " instances.");
}