BuddyPress.org

Your question pertains to bbPress, not BuddyPress.
The bbPress support forum is here.

Anyhow…
The filter hook you want is in bbpress\includes\common\functions.php

apply_filters( 'bbp_forum_subscription_mail_message', $message, $topic_id, $forum_id, $user_id );

function jorrit_filter_forum_topic_message_email( $message, $topic_id, $forum_id, $user_id ) {

    $message = wp_trim_words( $message, 15 ); // change 15 to however many words you want

    return $message;
}
add_filter( 'bbp_forum_subscription_mail_message', 'jorrit_filter_forum_topic_message_email', 10, 4 );