I am having Apache Web Server as the web server and NodeJS is also running on the same system at a different port. I am reverse Proxying it in order to connect to it and utilising it for different purposes. I would like to know what are the various ways in which this kind of architecture could be scaled up for 10 Million users or so, Is Apache a good choice to go with. If yes please let me know how to manage such a scenario and because I could not go ahead with different architecture.
If you throw enough resources at it (more and more instances behind a load balancer), of course Apache can take it. That's the philosophy of pretty much all web applications: scaling horizontally. But since it creates a new thread for each request, it's efficiency (per instance) is quite low. If it takes you x instances of Apache to sustain your traffic, you can probably switch to nginX(which uses the much more efficient evented model, as opposed to Apache's threaded model) and sustain the same traffic with, perhaps x/2 instances.
nginX has tradeoffs too: far fewer support resources, less uptake in the enterprise, hasn't been around as long as apache, need to recompile if you want to add a new module, etc. It is up to you to decide if it is worth it. Smaller/leaner companies prefer nginx to reduce costs, while big enterprises tend to prefer apache becuase of more support and people who know to configure it.