It really depends on the losses you are willing to sustain. Because combat is done in rounds, you can use smaller fleet, but you will have more casualties.
We discussed this few weeks ago and it was determined by Devs that you would need 120 dreadnoughts to take over a planet with no casualties (if you are at peace with the target and have 0 skills trained that increase dmg)

Also the 18 value is correct, colony has 18 hp/def per 1mil population.