How many printers can I handle with one Repetier-Server installation?

There is no limitation by Repetier-Server software, so theoretically you can control any number of printers at the same time with one installation. However, the actual number of printers that a PC can handle simultaneously is limited by the hardware.

With a Raspberry Pi 3 we could handle 5 printers at the same time without problems, with a simple computer (Intel i5) over 10 printers.

Please note that webcams can reduce the number of printers, especially if they have high resolution or frame rate or no hardware compression for jpg output.