The number of lights you can connect to a single Philips Hue Bridge (v2) is currently 63. Note that other devices, like sensors and switches, got their own limits. Hence, the total number of devices (not only lights) are higher.
This number is despite that Philip's marketing material often mention 50 lights. The reason for this mismatch is most likely because Philips want to be on the safe side, considering factors like available memory in the bridge may also restrict the number of lights (having many Hue Labs formulas are known to take up memory space, as an example).
These limits, capabilities of the bridge, are not hardcoded. The may change with future firmware updates of the Philips Hue Bridge. Philips did provide a way to programmatically ask the bridge using it's API about its capabilities.
See your Philips Hue Bridge's capabilities by selecting Capabilities in sidebar, under the Status.
As seen in the screenshot, there are also limits for how many scenes, rules, groups, etc you may use. Note that a sensor that counts towards usage is not necessarily a physical sensor (like the Hue Motion Sensor), there are also "virtual sensors" (that can be configured and used in rules). A physical device may also contain multiple sensors. For example, the Hue Motion Sensor does contain sensors to detect movement, to measure light levels, as well as temperature.
Explanation of some acronyms used by Philips Hue: