Let's ask Mr. Owl: How many licks does it take to get to the max limit of WSUS 3.0?
Ok, I'm dating myself horribly here. For anyone that figured out that tagline, great. I'll see you in the pudding line at the retirement home. For the rest of you: It was an old cartoon TV advertisement for Tootsie Roll Pops. Oh well. I shall digress (because that's about the only thing I'm really good at it seems).
Brian Tucker posted a great blog article about setting up a SUP role on SCCM 2007 using WSUS 3.0 of course. Read Brian's post if you want the good stuff. The mention of how many clients you can support with a WSUS server node brought back a memory from a webcast session a while back, in which someone asked that very same question of the Microsoft wizard. Ok, the wizard is the one talking, but they have a few background guys fielding the chat questions. I'll paraphrase the answer and embellish it a bit (another thing I'm good at).
The party line is "20,000 or thereabouts". However, and this is a BIG however, the answer really is....
IT DEPENDS
On what? You ask? Well, on several factors, actually more than several, but I can't think of what comes after couple, few, several... Oh well.
Some of the obvious ones are NIC capabilities, LAN link characteristics, WAN links, switches, routers, fiber quality, rodents chewing on cables, cables crimped in door jams, cable terminators installed by idiots, a poorly maintained server, a poorly maintained network, a poorly maintained staff. I could go on, but those are the obvious items. The not-so-obvious items are: the size and quantity of updates (even with BITS, it doesn't really do much since BITS can't really "See" your network traffic loads, only what's happening with the local NIC(s) and such). Also, the mix of local versus remote clients matters (LAN vs WAN = switches versus routers, etc). There really is no one-size-fits-all answer for this question. The same formulaic response has to be applied to many questions like this (i.e. File server performance with respect to various client groups at various locations and times of day).
Ponder this for a moment (be thankful I didn't try to sneak the work pontificate in there): Supporting 10,000 concurrent clients from a single WSUS server might work for most Windows security updates, but what about Vista Service Pack 1, or Office 2007 Service Pack 1? If you knee jerked and said "no problem!" you're drinking too much. Remember, I said "concurrent". Anyone that's experienced the painful burn and itch of such an attempt knows you need to stage things, either by time blocking or by hierarchical distribution (aka dissemination). So, again, the answer is really "it depends".
If you're really bent on getting a factual-based answer, I'm sure you can do it. I don't have the patience (or time it seems), but maybe you do. If so, please feel free to post a feedback comment here for others to enjoy?