I see this as a challenge, definitely not an excuse. I have actually found many benefits to having larger maths groups. A big one being the amount of discussion that you can generate by investigating everyones thinking, as there are almost always different methods for solving the same problem amongst the group. Exploring each persons solution, or misunderstandings is helping us form a more consolidated understanding of the maths problems.
One issue I have found however is finding the time to work with learners in smaller groups who need to consolidate learning, or fill in holes that the rest of the group has already mastered. So for the past few weeks while my student teacher has been in the class on full control, I have consciously been taking the time to work with these kids in small groups or 1 on 1.
Rather than work in the usual spot on the mat with the whiteboard, I have just been sitting down next to them at their table and working with them there. I only learned this recently but our table tops are all whiteboards!!!. So it's been absolutely perfect for this.
Once my student teacher leaves, I will no longer have as much time to do this. However, I am going to try and find the time, perhaps in the afternoons to do this more. As the confidence it is giving those learners in the next group session is easily noticeable. It is also helping me form a better picture of what those holes and misunderstandings are, in my learners.