title: Firebase FCM Usage Limits
body: <p>I was wondering if there is a comprehensive list that privides details of the various Firebase FCM usage limits.</p>
  
  <p>I knew of the Data message payload limit of 4KB maximum. 
  and then recently I got some issue with my app and then figured it was due to another limit - "maximum of 4 different collapse keys per device".</p>
  
  <p>I also remember in one of the recent talks Google mentioned they have removed certain limits that they had previously like number of topics etc.. </p>
  
  <p>I am trying to get details of what are all the various limits that Google enforces on the FCM usage.</p>