The plural of platypus is "platypuses". The next most correct usage is "platypodes", but this form is rarely used today. As for the commonly used word "platypi",? Well, English is an evolving language, and common usage sometimes dictates a word's acceptance. So, using platypi may be fun and quirky, but strictly speaking; it's incorrect. And here's another interesting fact. A group of platypuses is called a 'paddle'. "I saw a paddle of platypuses swimming by". How cute is that?
The term 'platypus' is modern Latin used mainly by the scientific community to name things. It is derived from the Greek words platus+pous. Since it is a made-up word, there is no plural form of the word in Greek or Latin.
If we were to use a Greek construct for the plural for words ending in –pus/poûs; the word would end in 'podes'. So using a Greek construct for the plural for platypus would be 'platypodes'. 'Platypodes' was sometimes used in the past for the plural, but it is rarely used today. In third declension Greek, it is also acceptable to append an "es" to the end of the words ending in –pus/poûs to signify the plural. Then 'platypuses' becomes acceptable. But, strictly speaking, 'platypus' is not a Greek name; it is merely constructed from two Greek words.
If we were to use a Latin construct, the plural would be 'platypi'. This comes from the belief that it is a Latin second declension noun. But it is not a Latin word. It is a made-up word from two Greek words that were latinised.
Platypus is now an English word. So we could apply the English language rules of the plural form. Then, as with most other English nouns ending in –s, the plural for platypus would be 'platypuses' (such as bus–buses, walrus–walruses).
A group of two or more platypus is called a 'paddle'. Other less creative terms include just plain platypuses or platypus (used in a plural form).
But we need to put the record straight. The platypus is a solitary animal. It does not get together in groups. The only time more than one platypus would come together is for mating.