Is there a way to insert multiple rows into a table with default values for all columns?
Using generate_series() and ctes. Tested in rextester.com:

create table t
( tid serial primary key,
  i int default 0,
  name text default 'Jack'
) ;


with ins as
  (insert into t (i, name)               -- all the columns except any serial
   values (default, default)
   returning i, name
  )
insert into t 
  (i, name)
select 
  ins.i, ins.name
from 
  ins cross join generate_series(1, 9);  -- one less than you need




For the case when there is only one column and it's a serial, I see no way to use the default. Using the generate_series is straight-forward:

insert into course
  (course_id)
select
  nextval('course_course_id_seq')
from
  generate_series(1, 10);





If there are other, more "peculiar" default values, like a UUID function or the non-standard clock_timestamp(), the statement will have to be adjusted accordingly, like the serial case.