In Biblical Hebrew, the word זֶרַע ("seed") is very similar to its English translation: that is, the word can be either singular or collective. For example, the following verse is an example of the singular noun used in the singular sense, and the second verse following is an example of the singular noun but used in the plural sense.
Gen 4:25 (MT)
.וַיֵּדַע אָדָם עוֹד, אֶת-אִשְׁתּוֹ, וַתֵּלֶד בֵּן, וַתִּקְרָא אֶת-שְׁמוֹ שֵׁת: כִּי שָׁת-לִי אֱלֹהִים, זֶרַע אַחֵר--תַּחַת הֶבֶל, כִּי הֲרָגוֹ קָיִן
And Adam knew his wife again; and she bore a son, and called his name Seth: "for God hath appointed me another seed instead of Abel; for Cain slew him."
The following verse now provides for the same Hebrew word, but in the collective (plural) sense.
Gen 7:3 (MT)
גַּם מֵעוֹף הַשָּׁמַיִם שִׁבְעָה שִׁבְעָה, זָכָר וּנְקֵבָה, לְחַיּוֹת זֶרַע, עַל-פְּנֵי כָל-הָאָרֶץ.
of the fowl also of the air, seven and seven, male and female; to keep seed alive upon the face of all the earth.
The context of this verse is all animal life on earth (collective meaning). The word also occurs in the plural to refer to a plurality of seeds.
1 Sam 8:15 (MT)
וְזַרְעֵיכֶם וְכַרְמֵיכֶם, יַעְשֹׂר; וְנָתַן לְסָרִיסָיו, וְלַעֲבָדָיו.
And he will take the tenth of your seed, and of your vineyards, and give to his officers, and to his servants.
In the Hebrew Bible, the plural form of the trilateral stem זָרַע therefore refers to the little grains that are planted in the ground. In other words, the singular form of the triliteral stem refers to humans or animals (or even to the little grain), and so may be used in either the singular or collective sense; that is, the context determines if there is one or more. But in the plural form, the word refers to seeds, which are the little things planted in the ground and/or are eaten as food.
Before concluding, in the Book of Genesis there is one passage, where the use of זֶרַע (in the singular form) appears to refer to the Promised Seed referenced in Gen 3:15. The following verses provide the clue.
Gen 22:16-17 (MT)
וַיֹּאמֶר, בִּי נִשְׁבַּעְתִּי נְאֻם-יְהוָה: כִּי, יַעַן אֲשֶׁר עָשִׂיתָ אֶת-הַדָּבָר הַזֶּה, וְלֹא חָשַׂכְתָּ, אֶת-בִּנְךָ אֶת-יְחִידֶךָ. כִּי-בָרֵךְ אֲבָרֶכְךָ, וְהַרְבָּה אַרְבֶּה אֶת-זַרְעֲךָ כְּכוֹכְבֵי הַשָּׁמַיִם, וְכַחוֹל, אֲשֶׁר עַל-שְׂפַת הַיָּם; וְיִרַשׁ זַרְעֲךָ, אֵת שַׁעַר אֹיְבָיו
and said: 'By Myself have I sworn, saith the LORD, because thou hast done this thing, and hast not withheld thy son, thine only son, that in blessing I will bless thee, and in multiplying I will multiply thy seed as the stars of the heaven, and as the sand which is upon the seashore; and thy seed shall possess the gate of his enemies
In this verse, the word זֶרַע occurs twice: the first time the word זֶרַע occurs, the word is used in the collective sense, meaning all of Abraham's descendants; however, in the second instance, the word זֶרַע is used in the individual sense. We know this because the Hebrew word for enemies (אֹיֵב) is the last word in the verse, but this plural word contains the possessive suffix, which is in the masculine singular form. That possessive masculine suffix is the tell-tale that the reference to זֶרַע is therefore to an individual, who is a male. This promised "seed" to Abraham in the Book of Genesis now appears to be the same "seed" promised to the woman, who was named Eve.
Finally, the tell-tale indicator of the singular masculine possessive suffix to the plural word enemies (אֹיֵב) in this verse of Genesis is what the Apostle Paul was referring to in his epistle to the Galatians.
Galatians 3:16 (NASB)
16 Now the promises were spoken to Abraham and to his seed. He does not say, “And to seeds,” as referring to many, but rather to one, “And to your seed,” that is, Christ.
The Apostle Paul noticed the "jot/tittle" possessive masculine suffix at the end of the last word of the verse in Gen 22:17, which is how he came to the conclusion that the second use of the word זֶרַע in this verse was in reference to the promised seed, who was the Christ. In other words, without the tell-tale indicator of the possessive masculine suffix, one could debate the second iteration of זֶרַע was in reference to the seed of Abraham in the collective sense.