The Pi camera is a popular add-on for the Raspberry Pi boards, it connects to the MIPI CSI connector of the board, and offers better performance than USB cameras.
But we are now seeing computer vision applications using more than one camera with stereo vision, 360 degree camera setups, and so on. One way would be to connect a Pi camera, and use the USB ports for the extra cameras, but Canerdurmusoglu had another idea, and instead designed IVport V2 Camera Multiplexer Board allowing you to connect up to four official Pi cameras to a single Raspberry Pi board. The maximum number of supported cameras is 16 since several boards can be daisy- chained.
IVport V2 works with Raspberry Pi Camera Module V2 (8MP version), and you can switch between cameras using 3 GPIO pins. If I understand correctly that setup does not allows you to use all the cameras at the same time, but you can switch between them, and based on the video below switching can happen fast enough to show to camera side-by-side with very little lag. The video was done with the earlier IVPort board supporting the first Pi camera.
Source code and instructions can be found on Github.
IVPort V2 is sold on Tindie for $75, but you can save a bit if you only plan to use two cameras by going with the $45 IVPort V2 DUAL board. If instead, you have a bunch of older (V1) Pi cameras , IVPort can be purchased for $80.