debugger-batch-max-lines is a variable defined in `debug.el'.
Its value is

Maximum lines to show in debugger buffer in a noninteractive Emacs.
When the debugger is entered and Emacs is running in batch mode,
if the backtrace text has more than this many lines,
the middle is discarded, and just the beginning and end are displayed.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.