Does the spelling of 'officiel' change depending on whether the subject being spoken of is singular or plural?
Yes, if it's common gender there is no declension: 'En officiel mand' (an official husband). However, if it's neuter gender then it declines with a -t: 'Et officielt kone' (an official wife). When there is a plural, though, you decline it -e, in this case the l on 'officiel' doubles up: 'Officielle aviser' (official newspapers). You also use this -e declension when the adjective in between a definite article and the noun: 'Den officielle avis' (The official newspaper). And when the adjective follows a possessive pronoun: 'din officielle avis' (your official newspaper). These rules apply to all regular adjectives. You can find a table of the declensions and more information regarding irregular adjectives in the Tips Notes of this skill.