Being a landlord is similar to selling any sort of space (think airline, hotel etc). Having all your units full every month would be 100% utilization. No business runs at 100% capacity forever. You must make allowances for non-use along with costs associated with renting out the units.
If you plan on using an agent to rent out the units, you should plan on 10 months of revenue per year per unit. 1 month will go to commissions and 1 month is 30 days on market. If you get a multi year tennant, or rent the place without an agent, the additional revenue goes to your bottom line.
As the other agent suggested below, buying a rented out property gives you some initial clarity on rent rates and income streams. However, once those contracts are up it may be a different story.
Broker Baird & Warner