This depends on whether you’re connecting USB cameras or analogue cameras.
Connecting more than 3 USB cameras can be a little complicated; it is possible to connect large numbers although this does depend on your computer – you’ll find all the details here.
Analogue cameras require that a digitizer card is inserted into your computer. Each card can connect to 4 cameras. How many cards you can connect depends on how many PCIe slots your computer has, but if for example, your desktop computer has 4 spare slots then you could easily connect 16 cameras.