Opnsense is not like OpenWRT, it targets running on relatively powerful generic x86 hardware. Intel CPUs and networking hardware usually works best because of driver support on BSD, but it will work on others. I say "relatively" because even low power old embedded CPUs are more than enough to route at a gigabit or more with lots of firewall rules and services running. Opnsense's cousin Pfsense also has some support for ARM, but that version is only really available on their commercially supported hardware.
Most people either buy a generic box that can be had for ~$250, or recycle an old PC and stick in a network card. You can also buy commercially supported hardware for Opnsense or Pfsense's parent companies, though the value proposition isn't worth it for home users IMO as you will pay a steep premium versus loading up something yourself.
