Make another M file and put in it all the names of the 20 existing M files.
If you want them to start at startup, put them startup.min the startup directory (see doc startup).
If they have systematic names, you can put the following in a loop:
[y1, y2, ...] = feval(function, x1, ..., xn)
where functionis the line you are developing in a loop.
Edit: if M files are scripts and not functions, it is safer for future versions:
eval(s)
where sis the name of the script.