What’s the max number of antennas you can put on a PC before it stops improving the connection?

It really depends on your wireless chipset and the access point you’re connecting to. Most client chipsets usually support 2-3 spatial streams, but high-end access points can handle 6-8. So there's definitely a limit based on the hardware involved.

Have you considered using a long antenna that connects directly to your router? It might be more effective than just stacking antennas. I think they call that Ethernet!

Generally, one antenna receives data and one transmits, allowing for simultaneous operation. If you're looking into beamforming with multiple antennas—similar to what routers do—each antenna would need its own connection and drivers.