If you keep plucking your eyebrows will they grow back even thicker?

It will not grow back thicker, but will be more noticeable because when you pluck all your hair follicles from an area at once, when they grow back, they will all be the same lenght and that will make them look as if you had new thicker hair. If you start plucking your hair you will have to continue plucking it continually.

NOTE: never ever shave the hair of the area you want to remove hair from. That will make the hairs look even fuller and in greater numbers.