Use the iOS 12 UserNotifications API's (threadIdentifier, summaryArgument, summaryArgumentCount) in Titanium!

TitaniumiOS12UserNotifications.js

var win = Ti.UI.createWindow({
    backgroundColor: '#fff'
});

var btn = Ti.UI.createButton({
    title: 'Schedule Notification'
});

btn.addEventListener('click', function() {
    schedule('id_1', 'New Notification', 'Hey there!', new Date().getTime() + 3000);
    schedule('id_2', 'Another Notification', 'Aloooah!', new Date().getTime() + 4000);
    schedule('id_3', 'Anooooother Notification', 'Hola!', new Date().getTime() + 5000);
    schedule('id_4', 'One more okay?', 'Hello!', new Date().getTime() + 6000);
    schedule('id_5', 'This is the last one', 'Hallo!', new Date().getTime() + 7000);
});

win.add(btn);
win.open();

var acceptAction = Ti.App.iOS.createUserNotificationAction({
    identifier: 'ACCEPT_IDENTIFIER',
    title: 'Accept',
    activationMode: Ti.App.iOS.USER_NOTIFICATION_ACTIVATION_MODE_FOREGROUND,
    destructive: false,
    authenticationRequired: true
});
  
// An action that does not open the app (background action)
var rejectAction = Ti.App.iOS.createUserNotificationAction({
    identifier: 'REJECT_IDENTIFIER',
    title: 'Reject',
    activationMode: Ti.App.iOS.USER_NOTIFICATION_ACTIVATION_MODE_BACKGROUND,
    destructive: true,
    authenticationRequired: false
});
  
 // An action that does not open the app (background action), but lets the developer
// type in a text (iOS 9+)
var respondAction = Ti.App.iOS.createUserNotificationAction({
    identifier: 'RESPOND_IDENTIFIER',
    title: 'Respond',
    activationMode: Ti.App.iOS.USER_NOTIFICATION_ACTIVATION_MODE_BACKGROUND,
    behavior: Ti.App.iOS.USER_NOTIFICATION_BEHAVIOR_TEXTINPUT, // or: Ti.App.iOS.USER_NOTIFICATION_BEHAVIOR_DEFAULT,
    authenticationRequired: false
});
  
// Create an example notification category
var downloadContent = Ti.App.iOS.createUserNotificationCategory({
    identifier: 'DOWNLOAD_CONTENT',
    actionsForDefaultContext: [acceptAction, rejectAction, respondAction]
});

// Register for user notifications and categories
Ti.App.iOS.registerUserNotificationSettings({
    types: [
        Ti.App.iOS.USER_NOTIFICATION_TYPE_ALERT
    ],
    categories: [downloadContent]
});
  
// Monitor notifications received while app is in the background
Ti.App.iOS.addEventListener('localnotificationaction', function(e) {
    alert('event: localnotificationaction');
});

// Validate notification settings
Ti.App.iOS.addEventListener('usernotificationsettings', function(e) {
    Ti.API.info('event: usernotificationsettings');
})

// Monitor notifications received while app is in the foreground
Ti.App.iOS.addEventListener('notification', function(e) {
    alert('event: notification');
});

function schedule(identifier, title, body, date) {
    Ti.App.iOS.scheduleLocalNotification({
        identifier: identifier,
        alertTitle: title,
        alertBody: body,
        summaryArgument: 'My Summary',
        summaryArgumentCount: 2,
        threadIdentifier: 'my_thread',
        date: date,
        category: 'DOWNLOAD_CONTENT'
    });
}