There seems to be a maximum of 9, and the counter AiMesh uses to keep track of the total may be inaccurate. But reports say it would literally tell you that the maximum has been reached....
I can't tell whether you tried to add it Wireless or connected via Ethernet, but adding a node should work either way. So if you tried wireless try connecting via Ethernet and retry adding, or if you tried via Ethernet move it close to the primary router and try adding it wirelessly. (Then relocate to where you want it. I would say 90% of the time you can add a node via Ethernet already positioned where it's finally destined but maybe 10% I need to wire it to the router then move it once added).
