I recently bought a new trolling motor to mount on my 18 ft glass boat. I was wondering, how much wire should I buy? I want to have a sufficient amount but with the price of copper I do not want to buy a lot of extra.

I would measure it out, just get some string and and run it the way the wire would run then measure.  Times that by 2 and there is what you need.  i would say probably like 40 foot of wire 20 foot a piece for power and ground