How many different species of insects are estimated to exist on Earth?
According to current estimates, there are approximately 1.5 million species of beetles, 5.5 million species of insects, and 7 million species of terrestrial arthropods.

https://www.annualreviews.org/doi/abs/10.1146/annurev-ento-020117-043348
What is the difference between an insect, a beetle, and a terrestrial arthropod?
An insect, a beetle, and a terrestrial arthropod are all different types of animals with distinct characteristics.

An insect is a type of invertebrate animal that is characterized by having three body segments, six legs, and often wings. They have a hard exoskeleton and can be found in a variety of habitats around the world, including on land and in water. Examples of insects include bees, ants, butterflies, and grasshoppers.

A beetle is a type of insect that is characterized by having two pairs of wings, with the front pair being hardened into protective covers called elytra. Beetles also have chewing mouthparts and a segmented body with three main sections: the head, thorax, and abdomen. Examples of beetles include ladybugs, fireflies, and stag beetles.

A terrestrial arthropod is a broad term that refers to any type of invertebrate animal that lives on land, and which has a segmented body and jointed appendages. This includes a wide variety of animals, including insects, spiders, millipedes, centipedes, and crustaceans such as woodlice. Terrestrial arthropods can have a wide range of different body shapes and sizes, and they play important roles in many different ecosystems around the world.

In summary, insects and beetles are both types of terrestrial arthropods, but beetles are a specific type of insect with certain distinguishing features such as hardened wings. Terrestrial arthropods are a more general category that includes both insects and many other types of invertebrate animals that live on land.