ffmpeg – How to convert a complete directory of videos via a terminal on Ubuntu Server 18?

I have a directory and I wanted to convert all .mkv videos to .mp4.

The only problem I encounter is:

ffmpeg -f concact-i list.txt -c copy new.mp4

the only problem is that this way I get all the directory but it comes out in a single video.
I would like to get everyone and convert …