My short term counselling model consists of 12 sessions. I always agree a set focus with the client, this is done in the initial assessment. I let the client know that whatever comes up during the sessions I will always bring it back to the agreed focus as that is what we will be resolving.
If something comes up during the counselling that does not relate to the focus, I acknowledge that we cannot explore that and bring the counselling back to the focus. The reason for having a focus in short term counselling is to be able to resolve anything in a short amount of time there needs to be a focus.