An adjective modifies a noun.
What noun does the word "one" modify at John 10:29.
An adjective complements or defines more precisely a noun, per "The Elements of New Testament Greek" by Jeremy Duff, David Wenham:
"5.2 USE OF ADJECTIVES (1)
ATTRIBUTIVE The most common use of an adjective is called the attributive use. This is where the adjective
defines more precisely an attribute of one of the nouns or pronouns in a sentence."
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PREDICATIVE
"In these sentences it is important to understand that the adjective is not an object, but rather is an adjective qualifying the noun: ‘holy’ is telling us something further about ‘the law’. Thus
it needs to agree with the noun in gender, case and number. However, such use of an adjective is different from the attributive use, because the adjective is not merely qualifying one of the nouns in the sentence –
the whole point of the sentence is to make this description. The adjective itself completes the sentence (hence the adjective is called a complement). This use of the adjective is called the predicative use. To tell the difference between the attributive and predicative use, try deleting the adjective from the sentence. If it still makes sense, the adjective was attributive. If it doesn’t, it was predicative."
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So, in order to determine which noun this adjective is complementing, it is necessary to look at the sentence (or clause) in which the word is being used. Its case, number and gender MUST match the noun it complements (again per Duff/Wenham).
Besides biblehub's interlinear and Duff/Wenham's textbook defining g3762 as adjective, so does the interlinear found at:
https://thebible.org/gt/index
(Among a bunch more I found that also list g3762 as an adjective).
In John 10:29
3762 oudeis οὐδεὶς; no one
Adj-NMS is Nominative, Masculine, Singular.
So all that is needed to answer is to determine which noun is Nominative, Masculine, Singular within this sentence/clause and you'll have the answer.
Hint, it's the noun that's "greater than all". None match Him. And He and the Son are one.
Even though, once again, your question has two assumptive errors in it.
What noun does the word "one" modify at John 10:29.
There is no word "one" found in John 10:29. The Greek word is "no one", or none (for short). The word means "not one" in English. G3762 does not mean "one" nor do all adjectives "modify" nouns. Some complement them. In this case, completely rules out any other noun being greater than the Father.