The short answer
For a first licence: 12 double lessons of basic material plus 2 double lessons of class-B-specific additional material, so 14 double lessons of 90 minutes each. If you already hold another licence, it is only 6 double lessons of basic material plus the same 2 additional ones.
The number 14 turns up everywhere and almost nobody explains what it is made of. It is not a number of evenings but the sum of two separate blocks — and the second one is where test entries fail.
Basic material: topics 1 to 12
Twelve double lessons that are the same for every licence class: priority, road signs, speed, environmental awareness, what to do at the scene of an accident. Anyone who already holds a motorcycle or moped licence has heard them — which is why the basic block drops to six double lessons for an extension.
Additional material: topics 13 and 14
Two double lessons that exist only for class B. They cover what sets a car apart from every other vehicle — technology, loads, trailers, driving with passengers. You must have attended each at least once before you can even be entered for the theory test.
The usual stumbling block
Somebody who comes to theory diligently but never happens to land on an evening with topic 13 or 14 stands there fully prepared and still cannot sit the test. Our theory timetable shows the topic for every session — the two compulsory ones run in the fixed rotation.
A double lesson is 90 minutes
Counting is done in teaching units of 45 minutes, teaching happens in doubles. So 14 double lessons are 28 units and around 21 clock hours. Here they fall on Monday, Wednesday and Thursday evenings plus Saturday mornings; which sessions you attend is up to you.
Is there an order? No
The 14 topics run on a continuous loop. You can join on any evening and miss nothing, because topic 14 is followed by topic 1. Repeating costs nothing extra — a topic that did not land you simply hear a second time. What each one covers is on the theory topics page.
Attendance alone is not enough, though: the topics are the foundation, the test is still sat individually. What actually helps you pass the theory test is its own subject.