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I have come across 多き being used in combinations such as 恵み多き宝 or 恵み多き地. I believe the correct reading is おおき. However, I was not able to find an explanation to the differences, e.g. to 多{おお}い.
In brief, this is the Classical Japanese attributive form of modern 多【おお】い. The attributive form or 連体形【れんたいけい】 is the specific conjugation form used when an adjective or verb is used to modify a noun or noun phrase.
All modern -i adjectives have the same -i ending for both the 終止形【しゅうしけい】 ("terminal / conclusive form", the form used to end a sentence) and the 連体形【れんたいけい】 ("attributive form", the form used to modify a noun or noun phrase).
Examples:
In Classical Japanese and older stages of the language, the terminal and attributive forms were different: the terminal ended in ～し, and the attributive ended in ～き.
As an extra wrinkle, adjectives with modern endings of just ～い (so-called ク活用【かつよう】形容詞【けいようし】, or "-ku conjugation adjectives", named for the distinct adverbial ending, the same for both modern and Classical) conjugated slightly differently from adjectives with modern endings of ～しい (so-called シク活用【かつよう】形容詞【けいようし】, or "-shiku conjugation adjectives"). The key difference is that the terminal for -ku adjectives replaces the ～き with ～し, while the terminal for -shiku adjectives just drops the ～き (so you don't have doubled ～しし).
Examples:
