Translation:That plate is round.
It's most grammatically correct to use it for a singular noun, but Japanese plurals are always a murky area.
The plural form for demonstrative adjectives :
•この = kono →これらの = korerano = these + noun
•その = sono →それらの = sorerano = those + noun
•あの = ano →あれらの = arerano = those over there + noun
Ex. これらの国々 = korerano kuniguni = these countries
Note : Again, these plural forms are for formal use and we don’t use them so much in a daily conversation.
Plural forms in Japanese :
Unlike other western languages, we have limited plural forms for nouns.
Ex. 生徒 = seito = student(s) → (plural) 生徒達 = seito tachi = students
日 = hi = day(s) →(plural) 日々= hibi = days
人 = hito = person/people →(plural) 人々 = hitobito = people
We use both singular or plural form of demonstrative adjectives for those nouns in plural form
Ex. these people:
この人達 = kono hitotachi or これらの人達 = korera no hitotachi (not so common)
Ex. these children:
その子供達 = sono kodomotachi or それらの子供達 = sorera no kotomotachi (not so common)