1lb/450g plums is around 5 - 6 medium-large plums, depending on size. You can scale the recipe up as needed, they might just take a little longer if more, and it can be trickier to get a single layer which I think works best for cooking, though is not strictly needed. If you can't forma single layer, carefully rotate plums part way through cooking from top to bottom to help them cook evenly (even with a single layer, you may want to do this if they appear to cook quicker in one part of pot compared to another).