Size of keystack buffer?

Is there a limit to the number of keystrokes that can be fed to a program? I have this in the BTM:

keystack /W500 alt-n alt-f "%cPFC" alt-o alt-n alt-n end up alt-n enter /W5400 alt-x

Where %cPFC is a 60 char full filename - including path.
It seems to work until the third alt-n. I have even tried changing the 5400 to a higher and lower num,ber but that doesn't change anyhting.