For riding on the road, a single light mounted to your handlebars and seat post would be more than enough - mountain bikers often use a helmet-mounted light to illuminate where they are looking, but for road riding these aren’t necessary.
Many lights will come with a plastic/silicone mount, however, some of the more heavy-duty bike lights will have a dedicated mounting bracket, often made from aluminium for extra security.
With the advent of out-in-front computer mounts, there are plenty of options which see an action camera-style or brand-specific bracket on the underside; we like these as it cleans up your cockpit but still allows you to ride with a light.
If you’re commuting and you need to secure your bike outside with a bike lock, look for lights that can be quickly removed without a multitool.