The verb תִּהְיוּ (tihyu) is conjugated in the plural number, from the root form (lemma) הָיָה (hayah), “to be.” Since no subject occurs immediately adjacent to the verb, then it is assumed to be אֲתֶּם (attem), “ye”—the 2nd person, plural number, (masculine) personal pronoun, in agreement with the preceding masculine plural noun בְּנֵי־יִשְׂרָאֵל (benei Yisraʾel)—“the children of Israel,” to whom Moses was speaking.
קְדֹשִׁים תִּהְיוּ כִּי קָדֹושׁ אֲנִי יַהְוֶה אֱלֹהֵיכֶם
kedoshim tihyu ki kadosh ani Yahveh eloheikhem
In the first part of the clause, the verb תִּהְיוּ is functioning as a linking verb, connecting the implied plural subject אֲתֶּם (“ye”) to the predicate adjective קְדֹשִׁים (kedoshim)—“holy.” In Hebrew grammar, adjectives decline according to gender (male, female) and number (singular, dual, and plural).1 2 Hence, the adjective קָדֹושׁ (kadosh)—which is actually the adjective declined in the masculine gender, singular number (i.e., the lexical form), becomes קְדֹשִׁים (kedoshim), declined in the masculine gender, plural number, as indicated by the plural ending (ים-) and slight change in diacritics.3 Again, this is necessary so the adjective קְדֹשִׁים can agree in number (and gender) with the implied plural (and masculine) pronoun אֲתֶּם—which refers to the masculine plural בְּנֵי־יִשְׂרָאֵל (benei Yisraʾel).
In the last part of the clause, the adjective remains as קָדֹושׁ (masculine gender, singular number) because the subject of the implied linking verb אֶהְיֶה (ehyeh)—“[I] am”—is אֲנִי יַהְוֶה (ani Yahveh)—“I, Yahveh”—a masculine gender, singular number subject.
In summary, קְדֹשִׁים is used in agreement with a (masculine) plural subject, while קָדֹושׁ is used in agreement with a (masculine) singular subject, all according to the rules of Hebrew grammar.
1 Pratico and Van Pelt, p. 60, 7.2
2 They can also have absolute and construct forms when functioning substantively (as a noun).
3 due to propretonic reduction
Pratico, Gary D.; Van Pelt, Miles V. Basics of Biblical Hebrew Grammar. 2nd ed. Grand Rapids: Zondervan, 2007.