message-log-max

message-log-max is a variable defined in `C source code'.
Its value is

1000

Documentation:
Maximum number of lines to keep in the message log buffer.
If nil, disable message logging. If t, log messages but don't truncate
the buffer when it becomes large.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.3 of Emacs.