The factor that affects the number of residents in a PEC is the Capacity Modifier, a Nursing Homes [Option] that can vary the number of rooms from the default value of 250

But - because a room can be occupied by either a single Senior, or a married couple - the number of residents in a PEC will be 20% ~ 35% greater than the number of rooms. That means a 250-room PEC set to Capacity Modifier 'Realistic (x1.0)' could accomodate as many as 250 * 1.35 = 340 Seniors

And at each end of the scale:
'Give em room (x0.5)' will permit up to 250 * 0.5 = 125 rooms, * 1.35 = up to 170 Seniors
'Pack em like Sardines! (x3.0)' results in 250 * 3 = 750 rooms, * 1.35 = up to 1000 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 PEC 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 PECs will include Adults as well as Seniors.

As the vanilla Watchtower and Lookout Booth were generating far too many tourists, they have been replaced with the ploppable Watchtower and Lookout (Parklife DLC required).

And to further reduce traffic, and also remove the presence of residents' luxury cars from the carparks - the number of carparks has been reduced to one (for the use of the PEC's Controller).

Amended the Asset to operate as a Level '0' Nursing Home.
Added an additional image to demonstrate that the Asset functions in locations remote from the city, and also to incorporate ploppable Watchtowers.