Popularities
Vertices. Though, as a result of persistent misuse, vertexes is gaining some popularity.
It depends on the kind of popularity. There's good popularity and bad popularity.
POPULARITY
triceratops popularity
The pluralization of octopus as "octopi" is in fact, in its Latinate form, incorrect, as this would require it to be a second declension Latin word. Since its classification in Latin form is third declension, according to classical Latin grammar rules, the plural of octopus should be octopodes. In any case, the more commonly used plural of octopus is "octopuses" - which classicists regard as an incorrect oversimplification. Besides this, the plural of platypus cannot be universally agreed upon either - "platypuses", "platypus", and "platypi", as well as the grammatically correct plural "platypodes", are used with varying levels of popularity.
his or her popularity
The plural of 'this' is 'these' and the plural of 'that' is 'those'.
the popularity of Mexico is 5,009,803
Popularity is a noun.
the popularity in Kenya is 1%
Isthmi is plural. There are actually two plural forms. I will list them from singular to plural. Isthmus - Singular Isthmi - Plural Isthmuses - Plural
If by popularity, you mean score, yes. Only in a popularity contest.