I want to build a prototype for an intelligent traffic signal system for a four-way cross junction, hence I need to connect 12 led lights i.e. red, yellow, green and 4 cameras for taking input images of traffic and 4 LCD display or 4 2-digit seven-segment display to display timer. Can I connect all these to a single raspberry pi, or please suggest some alternative for the same.
cameras: yes if USB webcams, no if CSI cameras (official rpi camera module) - you can only use 1 on a Pi, or 2 on a compute module
LEDS: yes, easily (one per GPIO)
LCD display: it depends on the protocol. Some displays are SPI, some I2C. With these protocols you can share some of the pins, but others you need unique pins, so you would need to work this out and plan which pins you will need to use, and you may find you can make it work with the Pi's GPIO header.
Alternatively, a good solution could be to use multiple Pis (maybe even 4, then you can use the official camera module on each one) and network them together.
please suggest some alternative for the same.
You're fishing. This part of the question isn't appropriate for Stack Exchange. I'd suggest you do some research and either bring back specific questions, or you're probably best taking it to the raspberry pi forums.