How Many Anonymous Blocks Can Be Defined?

An anonymous block is stored in the user's current session without any name. So you can only define one anonymous block at any time. If you define another anonymous block, the new block will replace the previously defined block, as shown in the following script:
SQL> set serveroutput on;
SQL> begin
2 dbms_output.put_line('Hello world!');
3 end;
4 /
Hello world!
PL/SQL procedure successfully completed.
SQL> begin
2 dbms_output.put_line('This is a PL/SQL FAQ.');
3 end;
4 /
This is a PL/SQL FAQ.
PL/SQL procedure successfully completed.