Is it possible to connect many (20+) SPI sensors to an Arduino Mega?
You can control as many SPI devices as you can set the CS line for. You can even control all the CS lines from a SPI device. A SIPO register will help a lot there.

However you are probably better off using a proxy board to actually connect to the sensors and then connecting the mega to the proxy to communicate the measurements using a connection that is more robust over long distances.