How to run procedure in Oracle SQL developer
First, if you are creating a procedure in a package, the package name will need to be included when you call the procedure.

begin
  hotel.fill_city(10000);
end;
/


should correctly invoke your procedure.

Second, you have issues with the naming of your local variables.  Normally, you would not create local variables like city and postal_number that are the same as the names of columns in tables in your database.  That makes it far too easy to introduce errors in your code where you intend to refer to the local variable but scope resolution rules mean that you are really referring to the column name.  For example, if you write the perfectly valid function

CREATE OR REPLACE FUNCTION get_dname( deptno IN NUMBER )
  RETURN VARCHAR2
IS
  dname VARCHAR2(30);
BEGIN
  SELECT dname
    INTO dname
    FROM dept
   WHERE deptno = deptno;
  RETURN dname;
END;


in your WHERE clause, both references to deptno will resolve to the column in the dept table, not to the parameter deptno.  That means that no matter what value you pass in to the function, the SELECT statement will return every row from the table and, thus, throw a too_many_rows error.  Normally, you would come up with a convention on how to name variables and parameters that would not conflict with your table naming conventions.  Prefixing parameters with p_ and local variables with l_ is one common convention.  That turns our function into something like this 

CREATE OR REPLACE FUNCTION get_dname( p_deptno IN NUMBER )
  RETURN VARCHAR2
IS
  l_dname VARCHAR2(30);
BEGIN
  SELECT dname
    INTO l_dname
    FROM dept
   WHERE deptno = p_deptno;
  RETURN l_dname;
END;


The other option would be to use explicit scoping prefixes when referring to local variables (i.e. get_dname.dname and get_dname.deptno) rather than altering the names of your local variables.