For normal file browser page, the value is hard encoded in a javascript file. The file is compiled and minimised together with other js files. The result file is seahub/media/assets/scripts/dist/main.d43fb2e46e23.js for version 5.1.11 (for different version, the random number d43fb2e46e23 is different).
Open the file and search for string maxNumberOfFiles:500 . Replace the number 500 with a number you like.
For folder upload link page, the value is controlled by view_shared_upload_link.html#L107 as in the forum thread. Just replace the number 500 with a number you like.
In version 6.0, we will make the number configurable by a setting in seahub_settings.py