but how many subtitles and dubs can I have in one single .mp4 ?
.MP4 and .MKV are just containers, they can hold multiple media streams. You can convert with no quantity loss using ffmpeg.
https://askubuntu.com/questions/50433/how-to-convert-mkv-file-into-mp4-file-losslessly
Discussion
I didn't know if there are different limits in MKV and MP4 for number of streams. I've encountered both with multiple audio and sometimes video streams.