For a 60 minute homework, we recommend 3 topics per week on average for KS3 and 6 topics per week on average for GCSE.
If you set a shorter homework (e.g. 30 minutes) you may wish to reduce the number of topics so that students still have enough practice on each set topic.
You can use the Preview feature in Planner to see what questions will be set for different lengths of homework and numbers of topics.