Millennium, meaning a single period of 1000 years, has 2 l’s AND 2 n’s. That’s because it’s derived from the Latin words mille, meaning “thousand,” and annus, meaning “year” (not anus, which means “ring” or… something else. Or “old woman,” but that’s unrelated).
The plural of millennium is millennia, which also has both 2 l’s and 2 n’s. This is because millennium is a second-declension neuter noun in Latin, and the plurals of neuter nouns end in -a. Other examples of this that have been borrowed into English include datum (sing.)/ data (pl.), addendum/ addenda, memorandum/ memoranda, forum/ fora (though I don’t think anyone uses that plural anymore). (Incidentally, opera is the plural of the 3rd-declension neuter noun opus.)
It may be that the language is changing so that millennia becomes the singular form, and if/when it does I will be a relic as with the transitive lay/ intransitive lie distinction… but in the meantime I’m going to be an obnoxious borrowed Latin grammar enforcer.
Loki is the Philosopher King 