Sending notifications to a big amount of users


Lets say i have 50K users using my app. What would be the most efficient way sending notifications to all of them?

currently im storing in my MongoDB database a document for each user that has a user ID field and a registration token.

From the ionic documentation , the notification JSON file sent to ionic servers, has a field ‘Tokens’ that sends an array.

       "alert":"Hello World!",

How will it react when passing an array of 50K tokens ??

What is the proper way doing so?