@kYutobiI would strongly advise against using 18 as the minimum.
You should always use 12 or 24 as basic rates. 9,18,36,48 are not the typical rates in all devices.
As for the discussion: 12 Mbps is usually the lowest mandatory rate, going up to 24 could be OK too but does not have the most significant gain in airtime. Minimizing SSID's has a better impact.
If you're absolutely sure your design spec meets 24Mbps everywhere for the weakest device in your network, then sure go for it, but I suspect drops or issues around staircases and elevators if you do.