Is “popcorns” the correct plural form of popcorn?
Popcorn is an uncountable noun (it’s already plural, but treated as though it were a singular noun).
Native English speakers don’t use “popcorns”, unless they are talking about different brands/types of popcorn.
Although the above are possible, even so, most native speakers would probably use “brands/types of popcorn” in preference to the above examples.
Use of “popcorns” in the plural S form is rare.
If you want to use the word in a specifically singular sense, say “a piece of popcorn” or in the plural sense “pieces of popcorn”.
Since popcorn is collectively plural, one can only make reference to it in a plural or singular form by the smallest entity of which it is composed: the kernel.
Thus you can have a kernel of popcorn or many kernels of popcorn.
Popcorns would be a non-plural of popcorn, except perhaps as spoken by children of and under early primary school age.