The code below loops through specified number of topics in the forum. eg ‘posts_per_page’ => 100 .
Please the read the link included below.
if ( bbp_has_topics( array( 'author' => 0, 'show_stickies' => 'false', 'meta_key' => '_bbp_last_active_time', 'orderby' => 'meta_value', 'order' => 'DESC', 'post_parent' => 'any', 'posts_per_page' => 100 ) ) )
bbp_get_template_part( 'bbpress/loop', 'topics' );
Link to topic:
https://bbpress.org/forums/topic/display-list-of-topics-under-specific-forum/
you could specify ‘posts_per_page’ => -1
which will loop through all