There are over 250 references to this in the course explicitly tying God to the creations of many individuals. To refuse to accept this is simply unwillingness and denial. It means that oneness does not equate to "no individuals". It also means ACIM is NOT non-duality and we are not one big blob.
The information below is courtesy of Robert Perry.
The Course uses a number of terms to speak of the plurality of Sons.
The Course uses the term "Sons" 82 times. Simply using a plural term like this, of course, implies that there is more than one Son. Further, the Sons are often associated directly with Heaven, implying that they are not just post-separation illusions: "The Kingdom of God includes all His Sons" (T-7.XI.7:10).
The Course uses the term "children" to refer to the "children of God" 42 times. This, of course, is another plural term. It, too, is often used to speak of God's children in Heaven, not just on earth, thus making clear that it's not just a post-separation concept. For example, "God knows His children with perfect certainty. He created them by knowing them" (T-3.III.7:9-10).
Creation is a technical term in the Course, referring to an act that takes place solely in Heaven. The Course is clear that no creation takes place in this world: "In this world it is impossible to create" (T-17.IV.2:1). Interestingly, the Course uses the plural word "creations" 118 times, mostly referring to God's creations (though many times to our own heavenly creations). For instance, "In the creation, God extended Himself to His creations and imbued them with the same loving Will to create" (T-2.I.1:2).
The term "Sonship" is used 104 times. It is particularly significant because it at once captures both the unity and plurality of the Son. Merriam-Webster Dictionary defines the suffix "-ship" as referring to "the body of persons participating in a specified activity." The Sonship, then, is "the body of persons participating in the activity of being God's Son." It's clearly a membership concept. The Course refers to this, talking about "the members of the Sonship" (T-1.II.6:2). Further, the Course clearly links the Sonship with creation, which rules out the idea that the Sonship is a post-separation product: "The Sonship is the sum of all that God created" (T-1.I.19:2). "He [God] created the Sonship" (T-7.I.2:6).
There are a number of other terms the Course uses to denote the plurality of Sons. Parts: "the parts of God's Son" (T-20.V.1:5), "parts of the Sonship" (T-12.IV.6:8). Aspects: "all aspects of the Sonship" (T-13.VI.6:4). Extensions: "His [God's] extensions are like Him" (T-12.IV.6:8). Channels: "His [God's] communication channels are not open to Him" (T-6.V.1:5). Brothers: "In every case you have thought wrongly about some brother God created" (T-4.IV.2:3). Minds: "all the minds which God created one with me" (W-pI.156.8:6). God's Thoughts: "All His Thoughts are thus perfectly united within themselves and with each other" (T-6.II.8:1-2). Souls: This term was used copiously in the original dictation of the Course, and eleven references to it remain. It, of course, captures exactly the notion we are talking about, that of a God-created spiritual identity that is in some sense individual.