What are the server side requirements for installation QuickBlox?
QuickBlox can be installed literally on any hardware starting from the equivalent of AWS Small (1 GHz CPU, 1.7 Gb RAM, 160 GB storage, any Linux-based OS) but it depends on capacity you want to achieve. Under capacity we mean chats / API calls per second.
Please note that 'chats per second' would typically be less than the number of concurrent users as users only send 1 chat message every several seconds. There is almost no limit on concurrent connections, main thing is capacity.
Here are the Plans QuickBlox currently provide: https://quickblox.com/plans/