Hi emtie,
as I have the same demand currently I think https://rspamd.com/doc/configuration/options.html lists the options you are looking for. They are called history_file and history_rows
The latter can be found in /etc/grommunio-antispam/options.inc and the according override file would be /etc/grommunio-antispam/local.d/options.inc
Alternatively you can also set the number of history rows in /etc/grommunio-antispam/local.d/history_redis.conf according to https://rspamd.com/doc/modules/history_redis.html
Don't forget to restart grommunio-antispam after your change with systemctl restart grommunio-antispam