How do I know which volume of oxidant to use?

The general rule of thumb is the following:

• 10 volume : lifts 1 shade
• 20 volume: lifts 2 shades
• 30 volume: lifts 3 shades
• 40 volume: lifts 4 shades

Be realistic! If you want to lighten more than 4 shades, it must be done in multiple steps and may require bleaching firstly. Discuss with the cosmetician at the pharmacy firstly.