Files, as in how many MP3 files are in the folder, like so;



In that folder you can see 13 files, as in 13 individual mp3 tracks.

Why Media Player isn't playing all the files tho is odd. When you select play all from the left panel pane in the folder what happens? What do you see listed in Media Player? Does it show all the song titles in the right pane of Media Player? Like so;



I can't see it being down to anything like ID3 tags, as even if you have no ID3 tags Media Player still plays everything, just usually in alphabetical order rather than ID3 order.

Try this, highlight all the files in the folder, and right click on one (so that they all stay blue. From the drop list you get from right clicking select PLAY, and tell me what happens....