/Nine-ess/ or /Nines/?
Dear Inquisitive Idiot,
The former. A bit of a strange 'nym, I know, so I'll save you the trouble of waiting another 100 hours to ask where it comes from. 9S is one of the major characters of the 2017 JRPG NieR Automata and one of my top fictional characters. The full designation is YoRHa No. 9, Type S, but since that's something of a mouthful, he and the other androids which comprise the YoRHa forces are often referred to simply by number and letter. The number designates the android's personality type and the letter designates the model type; 9S is a Scanner model, so he's programmed to be highly curious, analytical, and inquisitive, to assist in his task of scouting enemy lines and gathering intel and so on. Anyway, that's why the usual pronunciation is "nine-ess." He does claim at one point early on in the game that most of his friends call him "Nines," but I think that's mostly a ploy to get his assigned partner (and love interest) 2B to call him that. She initially refuses, but much to her chagrin, it slips out a few times anyway.