Why does DBMS_SESSION.session_trace_enable create multiple files?

I have a trace on a session configured like this:

execute immediate 'ALTER SESSION SET TRACEFILE_IDENTIFIER = ''TEST1''';

DBMS_SESSION.session_trace_enable (waits => TRUE,
                                   binds => TRUE,
                                   plan_stat      => 'all_executions'
                                      );
-- do SQL stuff --
DBMS_SESSION.session_trace_disable();

When completed executing, I have some 80 .trc files created. Why is that, and is there a way to force all tracing to one file? Thanks.