One of the network sites I was on and reading about the other day laid out it's requirements (I don't remember the exact site or the exact figures, but these sound about right)
1,000 subscribers
15,000 video views a month
...but then, in italic print it said "unless you make great content, then we're willing to bend the rules" or something like that.
They (most networks) give their recruiters the freedom to use their own best judgement, to an extent.
Okay, I'm off to my bookmark tab to find which one. lol
One minute later... (edit)
Here. TGN's partnership requirements and stuff. Hope this helps.
http://tgn.tv/partners/
Another edit: (haha)
I just noticed, they (TGN) don't say anything about subscribers at all. I'm sure that varies network to network though.