I give lessons to individual pupils or to groups of two pupils.
Individual extra lessons can be very effective because they provide an opportunity to deal with many topics in a short period with attention to the specific gaps in the pupil's knowledge and skills.
Sometimes it may be more effective to work with a small group because the pupils may be able to motivate each other, to offer different perspectives on understanding or solving problems, or they may stimulate each other, in their own words, to obtain more insight in the mathematical problem with which they are confronted.
Learning to listen, to ask questions and to formulate solutions are often important steps in the development of self-confidence, through which pupils are able to deal better with the learning material in class or during group work at school.
If I give extra lessons to two pupils together, I only do so if they are friends or classmates, if they are in the same year and if they make use of the same learning method/textbook at school.