Accepted answer
Nine sessions a week against 1,100 kcal a day — 7700 kcal for the week — and at that intake the binding constraint is almost never the session count. Protein at 4 kcal per gram is the reason: 100 g a day is 400 kcal, 36 per cent of 1,100, and 2800 kcal of the 7700 for the week spoken for before anything else is eaten. 9 sessions also has to be recovered from on 1,100 kcal, and recovery is the first thing an energy deficit takes. Load matters more than frequency here: the stimulus that preserves mass is mechanical tension, so keep the weight and cut the volume rather than the reverse. None of this is medical or dietary advice.
Start with frequency and total weekly sets per muscle group, because those two variables explain most of the outcome and everything else is refinement.
Load is the retention signal. Reducing weight and adding repetitions preserves the workout and weakens the stimulus, which is exactly backwards for this objective.
Concretely, beginners retain the ability to gain muscle in a moderate deficit for several months; trained individuals should expect maintenance and be satisfied with it.
Load-matched comparisons show that maintaining intensity preserves strength and lean mass in a deficit better than reducing load and increasing repetitions.
One to three reps in reserve. Failure costs more than it returns here.