The maximum negative eye power is typically around -20 diopters. This means that if someone has myopia of -20 diopters, they will need to wear glasses or contact lenses with a power of -20 in order to see clearly. However, it is important to note that not everyone with myopia will need this level of correction, and most people with myopia will have a prescription that is much lower than -20 diopters. It is also worth noting that visual acuity between 20/200 and 20/400 is classified as severe vision loss, and anything below 20/400 is considered profound vision loss.