You may hear many bats at one time. You never know where you may find bats. Where I live, there are some trees, but lots of blacktop and concrete. I often see bats flying by at night. It is surprising the places bats turn up and the only way to know is to go out looking and listening for them.
Bats are most active after it gets dark.
I have found that about 3 hours after sunset to about 6 hours after sunset offers the greatest activity. However, going out to find and listen to bats just 15 to 45 minutes after sunset allows you to still see them against the early night sky. Seasonwise, you won't hear bats during the winter if you live in a colder area. Bats hibernate during the winter and are active during the Spring, Summer and very early Fall. If you are lucky enough to live in a warmer area with mild winters, you'll find bats are active all year.