I have a small app that has more than 300 subscribers, maybe up to 900 subscribers. I'm planning to implement real-time product info and came across pusher.com. It was easy to implement but I couldn't clearly understand its pricing. Pusher's startup plan is $49 per month and provides 500 max connections. I could not afford the Premium plan ($499 per month). If 900 users simultaneously logs in to my app and if I want to give each one of them an update, what are my options?
Disclaimer: I work at Pusher
Your understanding of concurrent connections seems to be correct:
If 900 premium users simultaneously logs in to my app...
This would indeed constitute 900 concurrent connections but 900 concurrent connections would not require a premium plan. On the same page you linked, you should be able to see the Pro plan which offers 2000 concurrent connections for $99 a month.
I would say that the likelihood that all of your users will be using your platform at the same time is probably quite low. If you currently have 300 subscribers, you could try the sandbox plan (which is free) or the startup plan which you mention, and see what the real world usage is really like. You can always upgrade if your usage is more than you expect.