You can daisy chain displayport, as long as you have the bandwidth for it. I think the real issue is not how many monitors, but just how many pixels you're trying to drive. Since secondary monitors are typically 1080p, the whole collection of them should be the same as a 4k main monitor.

USB video dongles.

My laptop and a thunderbolt dock can do 4, a 4k panel, 2 qhd and a 1080p all at 30fps

Every Screen adds on bandwidth needs, the more pixes and faster refresh, the fewer screens you can handle.

You can get there with multiple gpus, or if you have lower demands, usb->html

Good grapics cards have 3, you're not limited to one graphics card

You need a graphics card or smth to even have the ability to have >3 displays. Some Macbooks have a 2 screen limit, so if you want to use only external monitors you have to close the lid. PC graphics generally allow at least 3 screens. But thunderbolt usb is an option.