Hello. Through my mikrotik I need to know how many wi-fi devices are within reach of my router. Is this possible?
That would be MT router model RBMagicRouter.5hld. It comes with a crystal ball attached to the top. 
The best you can hope for is
a. scans for other APs in the area or anything else that is actively tx on wifi. (believe that is a function on all MTs with radios
b. lists active connections (so that you can see who is connected to your networks).
If you want a device that knows about every wifi capable device in a radius from the device, suggest contact the NSA.
/interface wireless scan wlan1 background=yes
Actually, there is “wireless snooper”, which can show all devices communicating around - not just AP but also clients connected to different AP!
However, it will not show wifi devices which are not communicating (what a surprise, right?)
Wireless scanner works well. It’s under the wireless tab in Winbox.
Whichever tool you use, I suggest NOT using your AP as it’ll drop current clients. Your kids won’t like you 
Use a spare router.
Ahs yes, snooper is good IMHO and gives a loooot of information.
Sent from my cell phone. Sorry for the errors.