bbPress.org

“The problem is, there is no template-tags.php”

It was renamed to just template.php

bbpress/includes/forums/template.php

However you should be able to use the following filter in your functions file, just amend the 100 to whatever you want

function bbp_increase-forum-per-page( $args = array() ) {
$args['posts_per_page'] = get_option( '_bbp_forums_per_page', 100 );
    return $args;
}
add_filter( 'bbp_before_has_forums_parse_args', 'bbp_increase-forum-per-page' );