A 3- to 4-foot tank or larger will require two to four bulbs overhead, though you can get by with less if you choose to use only lower light and/or floating plants, which will linger near the surface.
If your tank came without lights, buy a fixture suited for your tank, and if you have an option, get the fixture with room for more bulbs (better to have more than less). A 3- to 4-foot tank or larger will require two to four bulbs overhead, though you can get by with less if you choose to use only lower light and/or floating plants, which will linger near the surface. For smaller tanks, a single fluorescent tube will likely work well for a normal 2-foot tank or less, as they are more shallow.