I just want to preface this with I don’t work for Twitch, this is based my experience with the Helix API, and my own opinions.
There was some discussion about where to put the follower count over in the TwitchDev server on the desktop app not long ago. It discussed including the total in the /users endpoint response or int the user/folows endpoint response.
The result, or at least what I got from it, was that they wanted to keep the total in the /users/follows response because it can reflect a few different things: how many followers a channel has, how many channels a user is following, or whether or not a user is following another user.
One issue I see with being able to include multiple user_id’s is that the current response structure doesn’t currently accommodate that, and it would result in a major change in how the endpoint returns data. This is largely due the endpoint being designed around getting follow information about one user or the relationship between two users. There were a couple endpoints in the Kraken API that had different response structures based on your inputs, but I believe one design philosophy with Helix was to make a more uniform and consistent response structure regardless of inputs for a given endpoint.