You purchase a pen a month approximately, assuming you stick generally to the standard injection schedule. The pens get more expensive as you move up the doses, so it depends on which dose you settle on as a long term dose. I have done one pen at 2.5mg, then one at 5mg, then 7.5mg for the next 7 pens for example. I haven't had to buy a 10, 12.5 or 15mg pen yet.
If you shop around and change suppliers each order, you can take advantage of the new customer discounts, otherwise you are paying full price or sometimes companies offer a smaller discount for continuing customers. Or sometimes a general discount code, there were a lot of those in January for example.
You can get an idea of the range of prices and discounts on this price comparison website:
https://monj.co.uk/discount-mounjaro-price-list/
This is the full price (no discounts) list from the same website:
https://monj.co.uk/mounjaro-price-list-non-discounted/
I probably paid a bit over what I could have for my first few pens, but I was happy with the provider I had chosen and was reluctant to change. But I have recently changed to a new supplier and will do again to get a better price if needs be.
Some providers like MedExpress have referral codes you can try to give out to people, and if they use your code you get a discount (£40 for MedExpress atm) but obviously that depends on people using your code when they sign up as a new customer. So it's not a guarantee or predictable as to whether anyone would use it.