How much biodiversity need to us?

You mean why is biodiversity important to us?

Biodiversity boosts ecosystem productivity where each species, no matter how small, all have an important role to play. For example, A larger number of plant species means a greater variety of crops. Greater species diversity ensures natural sustainability for all life forms.

Biodiversity is the  degree  of  variation  of  life  forms within  a  given  ecosystem. The word 'biodiversity' is a contraction of biological diversity. Diversity is a concept which refers to the range of variation or differences among some set of entities; biological diversity thus refers to variety within the living world. The term 'biodiversity' is indeed commonly used to describe the number, variety, and variability of living organisms. This very broad usage, embracing many different parameters, is essentially a synonym of 'Life on Earth'. Maintaining the balance of ecosystem: -recycling & storage of nutrients, combating pollution & stabilizing climate, protecting water resources & protecting soil & maintaining eco-balance. It is a measure of the variety of organisms present in different ecosystems. This can refer to genetic variation, ecosystem variation, or species variation (number of species) within an area, biome, or planet.