The Parse open source server does not contain push notifications, yet.

While managing the servers, you will also have to look out the configurations.

Parse has suggested MongoLab for MongoDB server management. However, if you use MongoLab servers for database, you will end up paying for huge data transfer costs between MongoDB and parse server.

You shall create auto scaling groups to increase/decrease your web servers according to load. You will need a load balancer to handle this transition.

If you decide to go for your own MongoDB installation You need to create a cluster to manage high availability. Default MongoDB installation does not ask for password. It may result into unsecure configuration and openly accessible database. Need to manage database failover configuration, backups/restore and software/instance upgrades

If thousands of users are dependent on your service, you might want to implement a disaster recovery strategy.