What is the largest phylum of invertebrates?

Arthropoda is the largest phylum of invertebrates. But more than that, it is the largest phylum in the entire animal kingdom. 
According to the Encyclopedia Britannica, about 84% of all known animal species are arthropods, a group distinguished by a jointed exoskeleton of chitin -- armor made from a complex sugar that is excreted by the epidermis.
The basic body plan is modular. Typically, most body segments bear a pair of appendages, and the body segments are grouped into three function units: The head, the thorax, and the abdomen.
Arthropods share some characteristics with vertebrates, like bilateral symmetry. But they also differ in many ways. For example, they possess an open circulatory system, in which blood flows freely within the body cavity, rather than through blood vessels.
The phylum Arthropoda is currently divided into four subphyla:
Myriapoda (including centipedes and millipedes)
Chelicerata (including spiders, scorpions, ticks, and mites)
Crustacea, (including crabs, crayfish, lobsters, shrimp, krill, barnacles, and woodlice)
Hexopoda, (including insects, springtails, and other, insect-like relatives)
To date, most arthropod species identified by science are insects. Researchers have described approximately one million species so far, and some estimate that there may be as many as ten million insect species total. Thus, the total number of anthropod species must exceed this number.
Over all, arthropods are a very diverse and successful group. They have been found in almost every habitat, including marine and terrestrial ones. Some arthropods species have adapted to the extreme conditions of Antarctica. Others survive in the Namib desert.
https://www.britannica.com/animal/arthropod