Serviio

In Serviio 2.4, I've recently added a folder named "ZZ serviio-Test" to the shared folders. I've selected media type "Video", no display name, no retrieval of metadata, I've ticked the box "Use the poller mechanism for monitoring this folder", advanced settings doesn't contain any ignored extension.
Of the 16 contained videos, only 3 are listed when opening this folder in a renderer (e.g. my Samsung TV or BubbleUpnp on my Android phone). The videos with extension .mp4 are not listed. For other shared folders this is no problem, .mp4 videos are listed and properly streamed.
Is there an upper limit of videos for Serviio? For me exactly 5,500 videos are listed.
Removing folder "ZZ serviio-Test" and adding it again gives the same result. After deleting, the number of videos drops to 5,497 and goes to 5,500 after adding the folder.

Please help, I appreciate any advice.

If there's an upper limit to the file count, I'm not aware of one. Mine is currently showing 6999 videos, so beyond what you show.

Just because your files are in the MP4 container doesn't mean that they are playable on your TV or Bubble app. If you have the pro (paid) version of Serviio you could see if all of the files are listed in MediaBrowser. You could also load VLC on your Android phone and see if it shows them all. Usually files not appearing are considered unplayable by the detected device and won't show them. You could also look at the serviio.log file and see if it shows any errors when adding the files to the folder.