AiMesh is a free firmware upgrade for select ASUS routers that adds the ability to form multi-node networks. So you can start with an existing AC class router and add other copies of that router, or any other AiMesh-enabled router to your mesh.
Which routers are supported you ask? Well, the situation is fluid. The EdgeUp ASUS blog post dated today says
If you’re using the ASUS RT-AC68U, RT-AC86U, RT-AC88U, the RT-AC1900, the RT-AC1900P, and the RT-AC3100, RT-AC5300, or the ROG Rapture GT-AC5300, you can get started with AiMesh today. Click here to download the beta.
But if you go to the AiMesh page as directed, there are only the download links shown above for the RT-AC86U, RT-AC68U and RT-AC1900. The best thing to do is hit the support page for your router and check for a firmware version 220.127.116.11.384.1xxxx .
Be advised that, in typical ASUS fashion, AiMesh is a work in progress. So don't expect smooth sailing.