Here both manuscripts have the singular desire, but the 1830 compositor set the plural desires. In general, the original text allows for either grammatical number when referring to someone’s desire(s). We find the opposite number for desire(s) in another example of the phrase “to obtain one’s desire(s)” earlier in the book of Alma:
In this instance, the original manuscript reads desires and Oliver Cowdery changed the noun to the singular when he copied the text from 𝓞 into 𝓟. As discussed under Mosiah 18:10, 11, the critical text will in each case of desire(s) follow the number in the earliest textual sources.
Summary: Restore the singular desire in Alma 55:20, the reading of the manuscripts (“but he had obtained his desire”).