bp_get_the_thread_recipients() count is off when there are more than 3 recipients. Its counting the sender as a recipients. So, for example, if I send a message to 4 people, the displayed recipients says
“Conversation between 5 Recipients and you” instead of “Conversation between 4 Recipients and you”
Even if you changed ‘recipients’ to ‘participants’ it would still be wrong. Notice the “and you” at the end. If they change to ‘participants’ and removed ‘and you’ then you’re right, its no longer a count/math issue.