You didn't look very hard.

https://www.postgresql.org/docs/current/static/sql-copy.html

To copy into a file just the countries whose names start with 'A':

COPY (SELECT * FROM country WHERE country_name LIKE 'A%') TO '/usr1/proj/bray/sql/a_list_countries.copy';

So you would want COPY (SELECT ... LIMIT 100).

I'm assuming you want to copy between databases or via an intermediate file or something, otherwise you just want to use INSERT ... SELECT as shown in another answer.

You can use

INSERT INTO table2(col_list) 
SELECT col_list FROM table1 WHERE condtitions ORDER BY order_col_list LIMIT X;

You can limit number of rows when you copy (SELECT ...LIMIT) to, but not COPY from. You did not find such options in docs, because there are no such.

use insert into ... select from ... limit 100 instead