You need a ton of bandwidth and resources.
To calculate how much bandwidth you need, you will need to know your average bitrate for the video. Let's just say that your live video's bitrate is 1 megabit (ignoring overhead, retransmissions, sequences that require more bandwidth, etc.). Your 50Mbps only covers 50 users. That's 0.1% of what you require. You need 1,000 of those connections, to barely handle the load.
If you actually have a live event that 50,000 people will see, you no doubt have sponsors and should be able to afford a proper CDN. This isn't something you host yourself. You pay for a CDN so that capacity is available as you need it, and servers are close to your audience.
The best thing to do would be to get a YouTube account with live streaming, and let YouTube pay for the bandwidth.
Now, the protocol you use has nothing to do with what size of IP block you have. Those are unrelated, separate issues.
RTMP, RTSP, HTTP progressive, etc... if you use a CDN, you get to use all of them with little effort. You pick a streaming protocol based on device compatibility and capability.
Google Hangout works using WebRTC, which is primarily peer-to-peer. When you stream them to YouTube, there is a massive CDN that handles distribution in multiple codecs, multiple protocols, and multiple points of presence.