... How many people know of Aki-kun's trauma ... ?
... Is it only Akira himself and his daughter ... ?
... Or are there more ... ?
Which trauma? There are many. The most important ones are Sekai dying in front of him, and Sekai grooming him/sexually assaulting him. The first one many people know about (almost all the adults basically and a lot of other people) - granted, they don't know what Sekai really was to him.
If it's about the abuse then not a lot of people. Ami, for instance, doesn't seem to know - she thinks it was a love story that the world didn't understand (like her own, and to some degree that may have mirrored real Sekai's ideas about Akira, in that she may have truly "loved" him).
Maya may know to many extents, since he used her as a replacement - she knows at the very least they had a sexual relationship because her reasoning for separating Akira from Niki/Noriko was to separate him from the memories of Sekai. Osako, Wakana and maybe Imani infer that he was the boy, but again, they know only to an extent.
Niki may suspect, but I don't think she knows (although Niki has admitted she purposefully makes a point of ignoring a lot of things). Kaori has been given hints by Akira, but she's far from the realization of what she has inside of her and their relationship to wizard Akira.
So yeah, to most people Akira has been traumatized by seeing his brother and wife die in front of him in a car accident and that's it. And even the people that know or suspect of Sekai don't have the full picture. In that, Maya and Ami are the closest to knowing everything, with Maya on top.
Edit: Also, Maya Prime may have seen pictures of Sekai abusing wizard Akira in Scary Room. She's shocked and asks "is that her? but these should have been gone" - which are either Niki or Sekai, and she knows Niki/wouldn't have been surprised by Niki having pictures with Akira still.