SQL> create table test (
   2         test_id number( 9 )
   3       , constraint pk_test primary key (test_id)
   4   )
   5   /

Table created

SQL> insert into test (test_id) values ( 1 );

 1  row inserted

SQL> create or replace trigger taiudr_test
   2   after insert or update or delete on test
   3   for each row
   4   declare v_count integer;
   5   begin
   6     select count(*) into v_count from test;
   7     dbms_output.put_line('taiudr_test: count='||v_count||';');
   8   end;
   9   /

Trigger created

SQL> insert into test (test_id) values ( 2 );

insert into test (test_id) values ( 2 )

ORA- 04091 : table DAN.TEST is mutating, trigger/function may not see it
ORA- 06512 : at "DAN.TAIUDR_TEST", line  3 
ORA- 04088 : error during execution of trigger 'DAN.TAIUDR_TEST'

SQL> update test set test_id = - 1  where test_id  =  1 ;

update test set test_id = - 1  where test_id  =  1 

ORA- 04091 : table DAN.TEST is mutating, trigger/function may not see it
ORA- 06512 : at "DAN.TAIUDR_TEST", line  3 
ORA- 04088 : error during execution of trigger 'DAN.TAIUDR_TEST'

SQL> delete test where test_id =  1 ;

delete test where test_id =  1 

ORA- 04091 : table DAN.TEST is mutating, trigger/function may not see it
ORA- 06512 : at "DAN.TAIUDR_TEST", line  3 
ORA- 04088 : error during execution of trigger 'DAN.TAIUDR_TEST'