PG 10 streaming replication with repmgr basic cfg Q

Hi List,
I am learning to cfg PG 10 streaming replication with repmgr ( master
-> slave rep cfg) so far for basic small transactions , switching
over to slave and promoting as new master is been smooth , now I am
trying a copy/insert > 15 mill entries in a table in master rep server
using " copy" from a file after less than 10 min the I filled aprox
20GB allocated to log, wal_files, here is from my pstgresql. conf and
repmgr , I cfg PG10 to use replication slots do I still need
archive_mode "on" ?
Is this normal to generate so many wal_files ?
and
How I should handle "COPY" large amount/batch data in a streaming
replication cfg ?

my postgresql.conf
max_wal_senders=10
wal_level='hot_standby'
hot_standby=on ( on standby server)
archive_mode=on
archive_command = 'cp %p /var/lib/pgsql/10/archive/%f'

wal_keep_segments =10

max_replication_slots = 2
***************************
repmgr.conf

use_replication_slots=1

********************************
SELECT * FROM pg_replication_slots;
slot_name | plugin | slot_type | datoid | database | temporary |
active | active_pid | xmin | catalog_xmin | restart_lsn |
confirmed_flush_lsn
---------------+--------+-----------+--------+----------+-----------+--------+------------+------+--------------+-------------+---------------------
repmgr_slot_1 | | physical | | | f | f
| | | | 0/14000108 |

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