In terms of diversity, there are more than 1.5 million species of plants, animals, fungi, lichen, and bacteria documented on Earth. And we are discovering more species every day! In terms of mass, there are currently about 75 billion tons of living things (biomass) on Earth. And in the past there were even more living things than today. However, our planet has gone through a series of mass extinctions (due to meteor impacts, volcanoes, and/or other factors) that killed off many species (like dinosaurs). For instance, 250 million years ago 70% of land species and 96% of marine species became extinct.

Living things are hard to classify.Things like viruses are considered (generally) not to be living things, but they have RNA and can replicate themselves like many other things we do consider living. Also, the classification of living things will change the number. For example, there are many different races of humans. Do you consider each race to be a different living thing or the same? Similar for different types of cows, grasshoppers, and so on and so forth. Is your puppy a different living thing than the other dogs because it has its own personality? This question is very hard to answer, but you can be sure that there are too many to count, and certainly many, many, many that we don't know about.