The plural of apparatus is apparatus or apparatuses.
A set instruments, equipment or materials designed with a specific purpose in mind.
The word apparatus has a Latin root but unlike many nouns in Latin its plural is not formed by replacing the -us ending with an -i.
The plural for apparatus has two acceptable forms. It can remain unchanged, as in apparatus, or its plural can be formed in the regular English way of adding -es to the end of the word to form apparatuses.
The word apparatus refers to a collection of tools or instruments and so the plural of apparatus refers to different types of apparatus.
The word apparatus is much more commonly used than apparatuses in written English as you can see from the graph below. Apparatuses is rarely used.
Singular Example: Set the apparatus up on the desk in preparation for the experiment this afternoon.
Plural Example: Since there will be a number of different activities this afternoon, we will need to keep the apparatuses in separate corners of the hall.
The graph shows the occurances of the plural of apparatus in written English since 1800 using Google's Ngram Viewer.
The evolution of breathing apparatus has allowed firefighters the ability to move and interact with much more flexibility than in the past.