The factor that affects the number of residents in a Facility is the Capacity Modifier, a Nursing Homes [Option] that can vary the number of rooms from the default value of 100

But - because a room can be occupied by either a single Senior, or a married couple - the number of residents in a Facility will be 20% ~ 35% greater than the number of rooms. That means a 100-room Facility set to Capacity Modifier 'Realistic (x1.0)' could accomodate as many as 100 * 1.35 = 135 Seniors

And at each end of the scale:
'Give em room (x0.5)' will permit up to 100 * 0.5 = 50 rooms, * 1.35 = up to 54 to 70 Seniors
'Pack em like Sardines! (x3.0)' results in 100 * 3 = 300 rooms, * 1.35 = up to 400 Seniors

When the time comes to conduct a purge of Seniors, how many perish will be determined by how you change the 'Capacity Modifier' ingame. When you REDUCE the 'Capacity Modifier', the Facility will immediately reduce its occupants to match the new setting (and the population of the city will likewise be adjusted downwards by the same amount)

So, start off with the Capacity Modifier set to at least x1, so as to give yourself room to reduce the Seniors population when their time comes. Then - when the purge has completed - recalibrate the Capacity Modifier back to a higher setting - rinse and repeat.

Because the Nursing Homes mod uses the vanilla Retirement age of 50 - if you raise a city's Retirement age to something higher than 50 with Lifestyle Rebalance Revisited - then a Facility will include Adults as well as Seniors.