You cannot execute it. It is a setting from CONFIG.SYS, not command (usable in normal DOS, not in DOSBOX). DOSBOX has internally FILES=100 (or some number like that, I do not remember).
So if you have a program complaining about about missing FILES=15, it is a bug in DOSBOX. What program is it?

One thing to try: Make a CONFIG.SYS on your mounted C drive with this line. It will not influence DOSBOX at all, but there are a few stupid programs which just directly check the text in CONFIG.SYS...

Mirek

I have a similar problem with the game pole position (team f1 in english). I run it once or twice and everything went fine. But since the third time the game complains that it need files=40. The trick with the dummy-config.sys doesn´t help.

I probably found a solution for this problem (for pole position it worx). I also had the "files=40" problem. But moving the game back to my c: -drive (where dosbox is also installed) did help. The game is not longer complaining about the number of files.

STONKEEP will not work because it says it need FILES=15 ?

@edward just type:

sk win95

and it should work