• A botanical garden or botanic garden is a garden dedicated to the collection, cultivation, preservation, and display of a wide range of plants labelled with their botanical names or It is an establishment where plants are grown for scientific study and display to the public.
• There are now estimated to be 1775 botanic gardens in 148 countries around the world with many more under construction.
  
•  Kew Royal Gardens in London, England has the world's largest collection of living plants.
  
• The number of species growing in botanical gardens worldwide is about 80,000.
  
• So,the correct answer is '80,000'.