Here we have another textual omission resulting from the eye of the typesetter skipping from one word to another. In this instance, the 1837 typesetter allowed his eye to skip from the first much to the second one. The 1908 RLDS edition restored the phrase, probably by reference to 𝓟; the 1920 LDS edition probably used the 1830 edition in restoring the phrase to the LDS text.
Usually the Book of Mormon text conjoins fasting with praying, especially when individuals are seeking help from the Lord. For a list of examples, see under Omni 1:26. In fact, earlier on in Alma 17 the text specifically refers to the sons of king Mosiah as having “given themselves to much prayer and fasting” (verse 3), so the conjunctive occurrence of “they fasted much and prayed much” in verse 9 is fully expected.
Summary: Maintain in Alma 17:9 the conjoined expression “they fasted much and prayed much” (the reading of the earliest textual sources).