I can't give you numbers for Windows, but it is higher than that.
There's a limit for amuled (the daemon) of about 1000 files due to limitations of the underlying library and I know of people reaching this limit. The monolithic version should support even more files, at least on Linux, but I personally haven't tested this.
The same applies for sources, I lack the practical experience. I guess you could reach a couple of thousands sources (I would estimate the maximum to be at 5000-10000), but I wouldn't recommend going that high. This would put quite some strain on your network connection.
I would suggest regarding 2000-3000 sources maximum as a practical limit, although more should be possible.