It's official — Amarbot has his own number. I did this because I was using him to send some monitoring messages to WhatsApp group chats, but since it was through my personal account, it would mark everything before those messages as read, even though I hadn't actually read them.
My phone allows me to have several separate instances of WhatsApp out of the box, so all I needed was another number. I went for Fanytel to get a virtual number and set up a second WhatsApp bridge for Matrix. Then I also put my profile picture through Stable Diffusion a few times to make him his own profile picture, and presto: Amarbot now has his own number!
In case the profile picture is not clear enough, the status message also says that he's not real. I have notifications turned off for this number, so if you interact with him, don't expect a human to ever reply!