Hint: In general, to know if any given word is a noun or adjective we just need to keep in mind that; - If the given term is something that can be described, or if an action verb can be acted upon, then it will be a noun. - Otherwise, if the term describes something, that is to describe the subject or the object, then the term is an adjective.
Complete answer: The original word meaning of ‘myriad’ by the Greek translation is “ten thousand” or it was commonly called “countless”. The definition used in English is that ‘myriad’ is a rather large quantity which is not specified, but is very large”. So we use it to exaggerate numbers or to refer to quantities we cannot count as the number of ‘stars’.
When this term 'myriad' was initially introduced it had been classified as a noun that implied "ten thousand" or a group of something that has such a large size. The general form of using myriads as nouns is ‘myriad of’. Over the years 'myriad' has also taken to be an adjective were the ‘of’ which was used in the noun form got omitted.
Examples to clearly explain the difference; - We can recognize the myriad of ways people contribute to non-governmental organizations. In the sentence above we have used the noun form ‘myriad of’ to highlight the number of ways. - The kids who are too poor are exposed to myriad disadvantages while applying to universities abroad. In the sentence above we see that ‘myriad’ is in the adjective form, this is used to describe again a number or quantity, but this word is often replaced with ‘countless’.
But the fact is that neither of the forms is wrong, both are approved to be used in the English language. So therefore we can use ‘myriad’ in an adjective form or as a noun form where; ‘myriad’ will be the adjective form and ‘myriad of’ will be the noun form.
Note: A term that has a similar meaning to ‘myriad’ is ‘plethora’. At first, the word plethora was incorporated into the English language to describe an abundance of something. Plethora first denoted an excess of either humour or blood, and then it denoted a harmful or unnecessary surplus.