Please give me a hint, why sometimes the nouns ending in a stretched i refer to singular and some times plural. I don't think the adjective is the only thing defining the number.
It's the type of the noun. In the fourth declension, the accusative singular and plural forms are identical and both have the ī at the end. Examples would be "gelte" for a lunar noun (geltī), "hontes" or "zaldrīzes" for solar nouns (hontī/zaldrīzī) and "Targārien" for a terrestrial noun (Targārī). You can tell they're fourth declension nouns because the last syllable contains and e.
"Jentys", on the other hand, is a solar noun in the second declension, and for those, the accusative singular ends in a simple -i, while the plural ends in an -ī.
If you want to learn more: https://wiki.dothraki.org/High_Valyrian_Noun_Declensions