Sorry I should add I'm just enquiring because I wondered if I might be able to give more info depending on what you're thinking about, but obviously only share what you feel comfortable with. I realise my above answer, while honest, is probably quite unhelpful at the moment!
It depends on the individual, their problem, the treatment protocol used by the therapist ('CBT' is the overarching term but it differs in practice depending on which disorder it's treating) and the service guidelines. Some areas of the country offer limited numbers such as only 7-8. Some will allow you to go up to 20. If you're having sessions with a PWP at step 2 you'll get 6 (they deliver cbt based guided self help but are not actual cbt therapists).
You'll need to ask your clinician at assessment for their best guess but it depends on how fast you make progress too. Usually you'll have a review several sessions in to see if it's helping and then agree whether or not to continue.
It depends how much work you put in between the sessions. It’s like seeing a personal trainer once a week but to see any results you need to train in the week on your own too. The CBT gives you techniques that then you need to keep on applying in your everyday life.
Spot on drink. If you do the in between session exercises set by you and the therapist you'll see results much much faster. If you're unable/unwilling to do work between sessions cbt won't work and will be terminated if you can't work together to find a way to enable you to do the work. It's like seeing a personal trainer, they can advise you what to do and encourage you but they can't do the work for you.
There are many therapies out there, and if you're paying privately you may have more choice in terms of which school of therapy you go for (cbt, person centred, gestalt) but if you're looking to have it on the nhs it's very hard these days to get anything but cbt or counselling.
In our area cbt is the first line treatment for depression and several anxiety disorders (gad, OCD, phobia, PTSD, social anxiety, panic disorder and health anxiety), and also for BDD. You may be seen one to one or offered a group. CBT doesn't work for every person in every situation but it's widely accepted as the best most evidence based therapy we've got right now for the above problems.
If you want to look further into it and do some reading let me know and I'll post some links to some great self help materials that will help you to understand cbt treatment and how it works. If you can let me know which issue you want cbt for that'd be helpful.