The number of proxies required to a complete a blast is not dependent upon the size of the blast to be completed. It is dependent upon the number of connections you'll be using.
You could complete a blast to one million blogs with one private proxy or fifty private proxies, and the success rate will be the same provided the number of connections is scaled appropriately.
A site that SB is capable of submitting a comment to, will always be marked as a successful submission as long as the proxy (private or public) used to connect to it is "up" (working) and doesn't have so many connections running through it as to make the load time of the page longer than that of the timeout settings you have set in SB's settings.
The reason success rates drop with public proxies and everybody says you should use private proxies to blast, is not because private proxies will create a successful submission on a site that a public proxy failed on...it's because private proxies are faster and have a nearly 100% uptime.
Public proxies however, are slow, have countless connections running through them at any given moment (yours and the hundreds/thousands of other people using that very same public proxy) & have a notoriously erratic uptime.
If SB goes to submit a comment with a public proxy that is "down", it marks that submission as failed, even if that site would have been marked as successful had the proxy been "up".
So, to answer your original question:
If you're using private proxies, a good rule of thumb to follow is to use as many proxies as it would take to make sure you've only got 5 "connections" running through each one. So if you want to submit comments in SB's fast poster mode using 50 connections, you'll need 10 private proxies (50/5=10).
If you want to submit comments in SB's fast poster mode using 100 connections, you'll need 20 private proxies (100/5=20).
Why the five connections per proxy you ask? Because you need to make sure that the amount of data your pushing through each connection isn't going to make the sites "timeout" before the allotted timeout time you have set in SB's settings. If the time it takes to load the page surpasses that, it is going to be marked as a failed submission.
So the real question you need to ask yourself isn't "How many private proxies are required to blast 50k comments" as you could do that with 1 private proxy and 5 connections. Your question should be "How many private proxies are required to blast 50k comments in X minutes" because that puts the onus on how many connections you'll be using, which is where the real problem could be.