I don't know if this theory has been discussed, but it's something I've noticed and thought I'd share with you guys.
Firstly, I think it's /extremely/ obvious that Eleven is not the only "experiment"or "test subject". Her number is 011, which could mean that there could possibly be over 100 other people just like her in that facility (or there could potentially be... we don't know what those government people had planned... ). With that being said, if there are others, and her number is 011, so that would mean there would be 10 people before her (001 – 010).
My following theory is, we've already met one of those test subjects before. Remember that guy was sent into the Upside Down? In the yellow suit? My bet is that he is, or was, one of those 10 test subjects. I say one of the 10 as he looks older than Eleven, which could mean he's been there longer. After he dies, Dr. Brenner says "my son". Sure, you could say it's his actual son, but Eleven calls Brenner "papa" and we all know that they aren't actually father and daughter.
What do you guys think? Is that guy from the start a "test subject" like Eleven?