instanceCountForMastersOnPage.js

var predicate = NSPredicate.predicateWithFormat("className == 'MSSymbolMaster'"),
    masters = context.document.currentPage().children().filteredArrayUsingPredicate(predicate),
    mastersLoop = masters.objectEnumerator(),
    master;

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

    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.");
}