THIS. I want to know as well. I have a back up iPhone 7 Plus and it does have 3 recents. I’m still on 12.4.1 though On that device. On my new iPhone 11 pro max has the same as yours
I’m on the latest beta and I still only ever get one contact showing up. And it’s not a contact I message the most. It’s clearly not working as intended.
I thought this took quite a while to populate - it needs you to send quite a few messages to people for it to work out your recents. So if you've set up a phone as new, then that list will be empty for a while. It certainly took my phone a week or so to settle with this, and for a while it had a "recent" of someone I'd not texted in over 6 months!