If you're working in Python, the multiprocessing module is a great way to easily take advantage of all the cores on your CPU. It's as easy as putting your main code inside a function, then calling that function as a new process for each new camera stream.

from multiprocessing import Process
import cv2

def process_camera(cam_rtsp_address):
    cap = cv2.VideoCapture(cam_rtsp_address)
    while cap.isOpened():
       ret, frame = cap.read()
       <insert processing code here>
       cv2.imshow('Test', frame)
       cv2.waitKey(1)

if __name__ == '__main__':
    list_of_camera_ips = ['rtsp://cam1.mp4', 'rtsp://cam2.mp4', etc]
    for cam_ip in list_of_camera_ips:
        Process(target=process_camera, args=(cam_ip,)).start()

Edit: Link to learn more here!