What is the % of carbamide peroxide needed for a teeth whitener?

Depends. Some products have 10%, others 16% and still others 37% carbamide perioxide.
Usually 5% to 35% Carbamide peroxide (5% to 35%) breaks down to hydrogen peroxide and urea and it is the hp that bleaches, whitens and decolorises the tooth when it breaks down to water and oxygen. This can leave teeth sensitive to hot and cold and remineralizing agents can seal the tiny pores in the tooth and root that were opened during bleaching and reduce sensitivity.