Hi Alan,
There's no limit (that I know of!) to the number (or total size) of files in a folder. The only limit is the 100MB for individual uploads. (And a somewhat larger limit of the amount of disk space on the server!)
So if one file is disappearing when you upload another then something weird is going on.
Note that if you upload a file which has the same name as another in the folder then it adds it as a new revision of that file, rather than creating a new one, so maybe this is what was happening? Have a look at the properties of the videos and see if any of them have unexpected extra revisions.
As an aside, the video with the sign falling over amused me. Apologies to everyone else here who doesn't have the required permissions to view this! ;-)
Thanks
Joe