Assuming an entirely digital setup, and the multiple cameras are viewed in the same way as modern times (a separate monitor per camera, or you have to select a camera to view its feed) a modern camera system would typically be a separate file per camera feed, written for a certain period of time before creating a new file with a new timestamp.
While a single file per camera feed would be nice, continuous reading and writing to an ever-enlarging file would cause slowdown due to manipulating a huge file, and it would make sense to perform archiving on small separate files periodically rather than archiving a file then trying to archive essentially the same information with a little bit more information added.
A more advanced setup, assuming the future of Shadowrun can allow for it, would be a single file with header information per data packet that identified not only the image being stored and the timestamp, but also the ID of the camera used to take the image.
Essentially, from a coding standpoint multiple files is an easier and more efficient system to make, as you only have to write simple code for one camera at a time, then store the information with a suitable filename to indicate which camera was being recorded; this also means you can perform smaller write operations more frequently asynchronously. A single file system would have to perform a single write action synchronously to a large file, which would produce a resource bottleneck that would make the system eventually crash due to out of memory exceptions.
If I were GMing your loop, I'd have a "multiple file" camera system, and you'd need to access the files for each camera feed for the amount of time you wished to loop; typically this would be a single file per camera giving you about an hour of looped feed, which would then loop until the people watching the feed noticed. In terms of difficulty/time taken though, the number of files wouldn't matter.
The process would involve editing code in the display element of the program to loop the video of each camera feed given a specific timestamp and apply a foreach to iterate through any and all cameras (or a switch-case statement if you had specific cameras you wanted to loop and not loop). If you rolled well, you'd also be able to subvert the recording and archiving element to write the looped feed to the new files, so there was no evidence you were there at all.