Can a motbike have 18 gears?

I can't really know what you mean by "motbike", but I'm guessing you mean "motorcycle", an engine powered two-wheeled vehicle. In which case I've never heard of a motorcycle with 18 gears.

If you're stubborn enough you could certainly build one, but it'd wouldn't be an improvement.

Or maybe you mean "mountainbike", human-powered pushbike. In which case it can certainly have 18 gears, or even more.