For the landlord units is elastic. If you have many units you can always not rent out a few, in fact you should always have a few units that you are remodeling and thus cannot rent out.

In general as a large landlord should have around 10% of units not rented, if you have more than that rented you should raise your rates until people go elsewhere thus bringing you down to 10%, while if you have less than 10% lower your rates until people start renting from you. Different landlords have different numbers, but 10% is a good starting place. 100 units at $900/month = 90,000, 90 units at $1000/month = $90,000, but you have a few units free in case someone desperate is willing to pay $1100/month (and of course you can remodel one of those empty units thus making it more desirable)