Help with light sensors

Buy light sensor I assume that you are referring to LDRs. Technically yes, it is possible to connect 50 light sensors to an Arduino but I wonder why anyone would do that. 

Few things to consider - are you trying to measure the intensity of light (Analog) or just check if it is total bright or dark (Digital). The problem here is Arduino UNO, for example, has only 14 digital pin and 6 analog pins. So you can only connect 14 digital sensor and 6 analog sensors to it. 

But if for some reason you want to connect 50 sensors then you should use a multiplexer or external ADC modules. The choice depends on your application requirement 

The VEX light Sensor allows the robot to sense the ambient light in a room. Unlike the Line Tracking Sensor, the Light Sensor does not generate any light, it only senses the amount of light already present in an area. The Light Sensor is an analog sensor, and it returns values in the range of 0 to 4095. Higher values indicate that the sensor is in a darker area, and lower values indicate lighter areas.

Some uses of the Light Sensor would be to have your robot stop moving whenever the lights in a room are swtiched off, or to sense when it moves into a shadow. When attaching the Light Sensor to your Robot, remember that it reads ambient light, so that it functions best when facing out into an open space. The Light Sensor can reliably sense light from up to six feet away from a light source. The sensor only senses visable light, and is not sensitive to infrared or ultraviolet light.

I hope you found your solution. 

A light sensor detects when a person or object moves in front of it and activates a light, which shines on a specified area. Sensor-activated flood lights help to keep the home safe and deter suspicious activity. Once the lighting system and sensor are installed, you might need to know how to configure the unit to suit your particular purpose. You also should program and test the system correctly before leaving your home for extended periods.

Twist the "Lite" and "Time" control knobs  counterclockwise until they each point to "Test." Twist the "Sens (Sensor)" control knob  to its middle setting. Turn on the floodlight switch and allow the light a minute to warm up, then turn off.

Adjust the sensor to cover the area you desire. Aim the sensor down to narrow the sensor area, or aim the sensor up to broaden the sensor area.

Twist the "Time" knob clockwise to lengthen the amount of time the floodlight remains on after the sensor is triggered.

The VEX light Sensor allows the robot to sense the ambient light in a room. Unlike the Line Tracking Sensor, the Light Sensor does not generate any light, it only senses the amount of light already present in an area. The Light Sensor is an analog sensor, and it returns values in the range of 0 to 4095. Higher values indicate that the sensor is in a darker area, and lower values indicate lighter areas.

Some uses of the Light Sensor would be to have your robot stop moving whenever the lights in a room are swtiched off, or to sense when it moves into a shadow. When attaching the Light Sensor to your Robot, remember that it reads ambient light, so that it functions best when facing out into an open space. The Light Sensor can reliably sense light from up to six feet away from a light source. The sensor only senses visable light, and is not sensitive to infrared or ultraviolet light. 

You can either use a VEX light sensor as suggested above or you can attain your requirement using a Analog multiplexers available like CD74HC4067 High Speed ​​CMOS 16-Channel Analog/Digital Multiplexer.Hope this helps!!!

An i2c bus allows to connect many devices in a single i2c port and you can access each ports using the bus address.
For this, you need i2c based light sensor.

For example, NOA1305 is a i2c based light sensor. Now you can connect as much leds as you want.

But drawback is the cost.

The light sensor circuit is a simple electrical circuit, which can be used to control the electrical load appliances like lights, fans, coolers, air conditioners, street lights,  automatically. By using this light sensor circuit, we can eliminate manual switching as the loads can be controlled automatically based on the daylight intensity. Hence, we can describe it as an automatic light sensor.

There are different types of light sensors available such as photoresistors, photodiodes, photovoltaic cells, phototubes, photomultiplier tubes, phototransistors, charge coupled devices, and so on. But, LDR is used as a light sensor in this light sensor circuit. These LDR sensors are passive and doesn’t produce any electrical energy.

But, the resistance of the LDR changes with the change in the daylight intensity. LDR sensor is rugged in nature, hence can be used even in dirty and rough external environments. Hence, LDR is preferable compared to other light sensors as it can be used even in the outdoor lighting of homes and in automatic street lights as well.