20 marks on the vernier scale allow for an uncertainty of ±0.05mm (0.005cm) because they allow you to visualize 20 spaces within the least count of 1mm (0.1cm)
10 marks on the vernier scale allow for an uncertainty of ±0.1mm (0.01cm) because they allow you to visualize only 10 spaces within the least count of 1mm (0.1cm)
this is still better than without a vernier scale where you can only visualize two spaces within the least count of 1mm (0.1cm) resulting in an uncertainty of ±0.5mm (0.05cm) for metric rulers that don't include a vernier scale.