Often I see ignorant skeptics of the Torah as using this argument to "prove" that Judaism adopted terms from polytheism, but this not at all the case.
The reason why Elohim ends with "-Im", (which usually implies a plurality) is because "-Im" is used in this instance in a respective, majestic form.
(the following is a rephrased summary from Isaac Troki's book against christianity, Chizuk Emuna chapter 9)
Genesis 1:1, "In the beginning Elohim [God] created the heaven and the earth." Elohim, ending in a plural form as though it meant Gods, has been interpreted by Christians as an evidence of the plurality in the Deity, consisting of the Father, the Son, and the Holy Ghost, who are denominated Trinity. The correct terminology of Elohim is as follows: —
Those who know the Hebrew language are aware that Elohim relates not merely to the Supreme Being, but also to angels and human authorities. Manoah, the father of Samson (mentioned in Judges 13:22), after he found that he had perceived "an angel of the Lord," said, "We shall surely die, for we have seen Elohim." In reference to human authorities, we read in Exodus 22:9, "Before the Elohim [judges] the cause of the two men shall be brought, and he, whom the Elohim [judges] shall declare guilty, shall pay twofold unto his neighbour."
We should like to understand how the name of Elohim.....in Exodus 7:1 "Behold I have made you an Elohim to Pharaoh," can be allowed by Christian expounders to allude to a plurality of persons, and represent in a mortal creature a visible Trinity?
The real object in the plural form in Elohim is to represent authority and power. This respective plurality is not just used for Elohim, but in words of profane signification. For an example, Adonim (lords) instead of Adon (lord). For instance, Isaiah 19:4, "In the hand of a hard Adonim [lord, literally lords], Genesis 39:20, "And the Adonai [lords instead of Adon, Lord] took him, viz., Joseph,"
The plural form is used instead of the singular in many modern languages (for instance you instead of thou).