The Septuagint LXX from academic-bible.com very distinctly shows different words between singular and plural. Here are some passages:
Gen. 12:7 – to your descendants (singular) I will give this land
καὶ ὤφθη κύριος τῷ Αβραμ καὶ εἶπεν αὐτῷ Τῷ σπέρματί σου δώσω τὴν γῆν ταύτην. καὶ ᾠκοδόμησεν ἐκεῖ Αβραμ θυσιαστήριον κυρίῳ τῷ ὀφθέντι αὐτῷ.
for I will give you and your offspring (singular) forever all the land that you see. 16 I will make your offspring (plural) like the dust of the earth, so that if anyone could count the dust of the earth, then your offspring (plural) could be counted.
ὅτι πᾶσαν τὴν γῆν, ἣν σὺ ὁρᾷς, σοὶ δώσω αὐτὴν καὶ τῷ σπέρματί σου ἕως τοῦ αἰῶνος.
16καὶ ποιήσω τὸ **
** σου ὡς τὴν ἄμμον τῆς γῆς· εἰ δύναταί τις ἐξαριθμῆσαι τὴν ἄμμον τῆς γῆς, καὶ τὸ σπέρμα σου ἐξαριθμηθήσεται.
Now the word of the LORD came to him: “This one will not be your heir; instead, one who comes from your own body[c] will be your heir.” 5 He took him outside and said, “Look at the sky and count the stars, if you are able to count them.” Then He said to him, “Your offspring (plural) will be that numerous.”
καὶ εὐθὺς φωνὴ κυρίου ἐγένετο πρὸς αὐτὸν λέγων Οὐ κληρονομήσει σε οὗτος, ἀλλ᾽ ὃς ἐξελεύσεται ἐκ σοῦ, οὗτος κληρονομήσει σε.
5ἐξήγαγεν δὲ αὐτὸν ἔξω καὶ εἶπεν αὐτῷ ᾿Ανάβλεψον δὴ εἰς τὸν οὐρανὸν καὶ ἀρίθμησον τοὺς ἀστέρας, εἰ δυνήσῃ ἐξαριθμῆσαι αὐτούς. καὶ εἶπεν Οὕτως ἔσται τὸ σπέρμα σου.
6καὶ ἐπίστευσεν Αβραμ τῷ θεῷ, καὶ ἐλογίσθη αὐτῷ εἰς δικαιοσύνην.
Then the LORD said to Abram, “Know this for certain: Your offspring (plural) will be foreigners in a land that does not belong to them; they will be enslaved and oppressed[
καὶ ἐρρέθη πρὸς Αβραμ Γινώσκων γνώσῃ ὅτι πάροικον ἔσται τὸ σπέρμα σου ἐν γῇ οὐκ ἰδίᾳ, καὶ δουλώσουσιν αὐτοὺς καὶ κακώσουσιν αὐτοὺς καὶ ταπεινώσουσιν αὐτοὺς τετρακόσια ἔτη.
On that day the LORD made a covenant with Abram, saying, “I give this land to your offspring, (singular) from the brook of Egypt to the Euphrates River:[f]
ν τῇ ἡμέρᾳ ἐκείνῃ διέθετο κύριος τῷ Αβραμ διαθήκην λέγων Τῷ σπέρματί σου δώσω τὴν γῆν ταύτην ἀπὸ τοῦ ποταμοῦ Αἰγύπτου ἕως τοῦ ποταμοῦ τοῦ μεγάλου, ποταμοῦ Εὐφράτου,
The Angel of the LORD also said to her, “I will greatly multiply your offspring (plural), and they will be too many to count.”
καὶ εἶπεν αὐτῇ ὁ ἄγγελος κυρίου Πληθύνων πληθυνῶ τὸ σπέρμα σου, καὶ οὐκ ἀριθμηθήσεται ἀπὸ τοῦ πλήθους.
I will keep My covenant between Me and you, and your future offspring (sperm) throughout their generations, as an everlasting covenant to be your God and the God of your offspring after you. 8 And to you and your future offspring(singular) I will give the land where you are residing—all the land of Canaan—as an eternal possession, and I will be their God.”
9 God also said to Abraham, “As for you, you and your offspring (plural) after you throughout their generations are to keep My covenant. 10 This is My covenant, which you are to keep, between Me and you and your offspring (sperm) after you: Every one of your males must be circumcised.
καὶ στήσω τὴν διαθήκην μου ἀνὰ μέσον ἐμοῦ καὶ ἀνὰ μέσον σοῦ καὶ ἀνὰ μέσον τοῦ σπέρματός σου μετὰ σὲ εἰς γενεὰς αὐτῶν εἰς διαθήκην αἰώνιον εἶναί σου θεὸς καὶ τοῦ σπέρματός σου μετὰ σέ.
8καὶ δώσω σοι καὶ τῷ σπέρματί σου μετὰ σὲ τὴν γῆν, ἣν παροικεῖς, πᾶσαν τὴν γῆν Χανααν, εἰς κατάσχεσιν αἰώνιον καὶ ἔσομαι αὐτοῖς θεός.—
9καὶ εἶπεν ὁ θεὸς πρὸς Αβρααμ Σὺ δὲ τὴν διαθήκην μου διατηρήσεις, σὺ καὶ τὸ σπέρμα σου μετὰ σὲ εἰς τὰς γενεὰς αὐτῶν.
10καὶ αὕτη ἡ διαθήκη, ἣν διατηρήσεις, ἀνὰ μέσον ἐμοῦ καὶ ὑμῶν καὶ ἀνὰ μέσον τοῦ σπέρματός σου μετὰ σὲ εἰς τὰς γενεὰς αὐτῶν· περιτμηθήσεται ὑμῶν πᾶν ἀρσενικόν,
For I have chosen[i] him so that he will command his children and his house after him to keep the way of the LORD by doing what is right and just.
ᾔδειν γὰρ ὅτι συντάξει τοῖς υἱοῖς αὐτοῦ καὶ τῷ οἴκῳ αὐτοῦ μετ᾽ αὐτόν, καὶ φυλάξουσιν τὰς ὁδοὺς κυρίου ποιεῖν δικαιοσύνην καὶ κρίσιν· ὅπως ἂν ἐπαγάγῃ κύριος ἐπὶ Αβρααμ πάντα, ὅσα ἐλάλησεν πρὸς αὐτόν.
But God said to Abraham, “Do not be concerned[e] about the boy and your slave. Whatever Sarah says to you, listen to her, because your offspring (plural) will be traced through Isaac. 13 But I will also make a nation of the slave’s son because he is your offspring (plural).”
εἶπεν δὲ ὁ θεὸς τῷ Αβρααμ Μὴ σκληρὸν ἔστω τὸ ῥῆμα ἐναντίον σου περὶ τοῦ παιδίου καὶ περὶ τῆς παιδίσκης· πάντα, ὅσα ἐὰν εἴπῃ σοι Σαρρα, ἄκουε τῆς φωνῆς αὐτῆς, ὅτι ἐν Ισαακ κληθήσεταί σοι σπέρμα.
13καὶ τὸν υἱὸν δὲ τῆς παιδίσκης ταύτης, εἰς ἔθνος μέγα ποιήσω αὐτόν, ὅτι σπέρμα σόν ἐστιν.
7 I will indeed bless you and make your offspring (plural) as numerous as the stars of the sky and the sand on the seashore. Your offspring (plural) will possess the gates of their enemies. 18 And all the nations of the earth will be blessed[h] by your offspring (singular) because you have obeyed My command.”
ἦ μὴν εὐλογῶν εὐλογήσω σε καὶ πληθύνων πληθυνῶ τὸ σπέρμα σου ὡς τοὺς ἀστέρας τοῦ οὐρανοῦ καὶ ὡς τὴν ἄμμον τὴν παρὰ τὸ χεῖλος τῆς θαλάσσης, καὶ κληρονομήσει τὸ σπέρμα σου τὰς πόλεις τῶν ὑπεναντίων·
18καὶ ἐνευλογηθήσονται ἐν τῷ σπέρματί σου πάντα τὰ ἔθνη τῆς γῆς, ἀνθ᾽ ὧν ὑπήκουσας τῆς ἐμῆς φωνῆς.
The LORD appeared to him and said, “Do not go down to Egypt. Live in the land that I tell you about; 3 stay in this land as a foreigner, and I will be with you and bless you. For I will give all these lands to you and your offspring (singular), and I will confirm the oath that I swore to your father Abraham. 4 I will make your offspring (plural) as numerous as the stars of the sky, I will give your offspring (singular) all these lands, and all the nations of the earth will be blessed[a] by your offspring (singular),
καὶ παροίκει ἐν τῇ γῇ ταύτῃ, καὶ ἔσομαι μετὰ σοῦ καὶ εὐλογήσω σε· σοὶ γὰρ καὶ τῷ σπέρματί σου δώσω πᾶσαν τὴν γῆν ταύτην καὶ στήσω τὸν ὅρκον μου, ὃν ὤμοσα Αβρααμ τῷ πατρί σου.
4καὶ πληθυνῶ τὸ σπέρμα σου ὡς τοὺς ἀστέρας τοῦ οὐρανοῦ καὶ δώσω τῷ σπέρματί σου πᾶσαν τὴν γῆν ταύτην, καὶ ἐνευλογηθήσονται ἐν τῷ σπέρματί σου πάντα τὰ ἔθνη τῆς γῆς,
and the LORD appeared to him that night and said, “I am the God of your father Abraham. Do not be afraid, for I am with you. I will bless you and multiply your offspring (plural) because of My servant Abraham.”
καὶ ὤφθη αὐτῷ κύριος ἐν τῇ νυκτὶ ἐκείνῃ καὶ εἶπεν ᾿Εγώ εἰμι ὁ θεὸς Αβρααμ τοῦ πατρός σου· μὴ φοβοῦ· μετὰ σοῦ γάρ εἰμι καὶ ηὐλόγηκά σε καὶ πληθυνῶ τὸ σπέρμα σου διὰ Αβρααμ τὸν πατέρα σου.
Isaac to Jacob
May God give you and your offspring (singular) the blessing of Abraham so that you may possess the land where you live as a foreigner, the land God gave to Abraham.”
καὶ δῴη σοι τὴν εὐλογίαν Αβρααμ τοῦ πατρός μου, σοὶ καὶ τῷ σπέρματί σου μετὰ σέ, κληρονομῆσαι τὴν γῆν τῆς παροικήσεώς σου, ἣν ἔδωκεν ὁ θεὸς τῷ Αβρααμ.
13 Yahweh was standing there beside him,[a] saying, “I am Yahweh, the God of your father Abraham and the God of Isaac. I will give you and your offspring (singular) the land that you are now sleeping on. 14 Your offspring (plural) will be like the dust of the earth, and you will spread out toward the west, the east, the north, and the south. All the peoples on earth will be blessed through you and your offspring (singular).
13ὁ δὲ κύριος ἐπεστήρικτο ἐπ᾽ αὐτῆς καὶ εἶπεν ᾿Εγὼ κύριος ὁ θεὸς Αβρααμ τοῦ πατρός σου καὶ ὁ θεὸς Ισαακ· μὴ φοβοῦ· ἡ γῆ, ἐφ᾽ ἧς σὺ καθεύδεις ἐπ᾽ αὐτῆς, σοὶ δώσω αὐτὴν καὶ τῷ σπέρματί σου.
14καὶ ἔσται τὸ σπέρμα σου ὡς ἡ ἄμμος τῆς γῆς καὶ πλατυνθήσεται ἐπὶ θάλασσαν καὶ ἐπὶ λίβα καὶ ἐπὶ βορρᾶν καὶ ἐπ᾽ ἀνατολάς, καὶ ἐνευλογηθήσονται ἐν σοὶ πᾶσαι αἱ φυλαὶ τῆς γῆς καὶ ἐν τῷ σπέρματί σου.