Are you looking at surveillance specifically or do you mean in general? As in, how many streams can Netflix, YouTube, etc. handle?
Your question is so broad, I am concerned about the quality and variance of answers that will be offered to it.
It depends on the storage server. I have seen smaller storage servers that are designed for home use only handle 6-10 simultaneous connections writing to it. On the other end of the spectrum a storage server that is designed off a PC spec could handle hundreds or thousands of simultanous connections. It really depends on the configuration and the data rate for each connection.
It's going to depend on what type of technolcogy you're using, the hardware under the hood, how much bandwidth, what you're read/write ratio is, what software you're using, etc. This is very complex design and cannot be answered in a few sentences.
I had the feeling that the limitation in the number of simultaneous connections was primarily imposed by the operating system used on the storage server. However, it seems like that this limitation is rather a design choice when implementing the storage server's software.
Well typically all your camera streams come to a single or multiple VMS servers and then those connect to your storage system. It's not like all your cameras are directly connecting to the storage system.
OK fair enough :) I don't really have any specific examples as the only external storage units I used were with OnSSI but again it depends on too many factors to give a general answer.