That is a very important question that no one can answer.

It's hard enough to figure out how many big, obvious things there are. There are somewhere around 10,000 species of birds, for example. There are around 5,500 species of mammals. Even with these species, we may not have found the ones that are small, rare, hidden, or look like other species.

Insects are a huge group. Scientists have identified nearly a million species, but there are probably many that we haven't identified yet. There are lots more other invertebrates, too. Worms, arachnids (spiders and mites), corals, and all sorts of other animals don't get as much attention, but there are lots of them.

I've only mentioned the animals so far. There are also plants, fungi, protists (lake amoebas and paramecium), and bacteria.

About 1.3 million living things have been named, but there's no way to know how many we haven't discovered. Some scientists estimate that this might be only 15% of the actual number of species.

The number of species there are is called biodiversity. Scientists like to look for patterns, so we wonder about patterns in biodiversity. Why do you think there are lots of species in the rainforest, but not a lot in the desert? Thanks for asking,

That is a VERY difficult question to answer, especially because there are still plenty of species left to discover (think insects & deep marine life)!! As of 2011, there were ~8.7 million different species recognized on Earth: ~6.5 million of these species live on land, and ~2.2 million of these species live in the oceans.

(Source: here).

If you include bacteria (as a more recent 2017 study did), that number gets MUCH larger - to the tune of ~2 BILLION different species (the majority of which are different bacteria; source: here ).

I realize that this doesn't directly answer your question, but for the majority of these species, we don't have an accurate count of their populations. Unfortunately, the species we do have good population data for are likely either at risk of becoming an endangered species, or are already an endangered species, possibly even on the brink of extinction (like the Northern white rhinoceros, which is extinct in the wild).

Biologists use several metrics to quantify "how many" organisms are in an ecosystem. One is the mass of the element carbon in those organisms [carbon being the basis for all known life. ]

This recent article gives what seems to be the current best-estimate of biomass on Earth at ~550 billion tons (550x10⁹ metric tons = 550x10^{^}12 kg ≈ 1,200x10¹² lbs). Another measure is a literal counting of every individual. Accurately doing this with all of the living organisms on Earth would be an impossible task. Instead, for such a large system biologists will extrapolate from a count of a much smaller region that is representative of the whole. The authors of the above article report (in their Supplementary Information, the "Abundance" column of Table S1, page 89) estimates of the number of organisms for each of the taxons of living things on Earth. Summing their totals for each taxon gives estimates of 10¹⁴⁷ or 10¹⁷⁸ (that is, 10 followed by 147 or 178 zeroes) living organisms on Earth. [The first number excludes viruses while the second includes them. Viruses are tricky because some do not consider them to be alive. While viruses meet some of the criteria scientists use to determine for life, they lack others. ]

Finally, the question may actually be asking about the number of different species on Earth rather than a direct count. This other article (also described in a news article here) gives a number of 8.7 million (8.7x10⁶).

Like the mass and count metrics used above, this is also an estimate which is impossible to prove in practice. [There is a more recent article which used scaling laws to reach a prediction that there are up to 1 trillion (10¹²) species of microbes alone. However, their lower estimate is "only" 100 billion (1¹¹), which is quite a large range.

There are also (math alert) claims that their methods are inappropriate and therefore these estimates should not be used. Assessing these arguments is beyond my current knowledge, but the authors defend their approach in a reply. ]

We don't know how many different kinds of living things there are on Earth - people keep finding new kinds - mostly tiny new kinds that are hard to see.

We don't know how many living things there are on Earth, counting the populations of all the different kinds of living things. But here's a link you could read, and add up the numbers for the different types of living things. For the most common species of bacteria, it says: "It is estimated that the oceans contain about 2.4 × 1028 (24 billion billion billion) SAR11 cells".
Source: here.

From ScienceLine:
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.

That depends on what you mean by "things". If you mean species (i.e. types of living things), then there are about two million species that we know about, but we estimate at least ten times that number that we haven't yet discovered. We don't know exactly.