The promises were spoken to Abraham and to his seed. The Scripture does not say, “and to seeds,” meaning many, but “and to your seed,” meaning One, who is Christ.
Textus Receptus (TR) - Galatians 3:16 >Τῷ δὲ Ἀβραὰμ ἐρρήθησαν αἱ ἐπαγγελίαι, καὶ τῷ σπέρματι αὐτοῦ. Οὐ λέγει, Καὶ τοῖς σπέρμασιν, ὡς ἐπὶ πολλῶν, ἀλλ᾿ ὡς ἐφ᾿ ἑνός, Καὶ τῷ σπέρματί σου, ὅς ἐστι Χριστός.