This is a difficult question to answer given it depends on a variety of factors (SAT/ACT scores, extracurriculars, community involvement, major/program you're applying into, demographics, family income, opinions of the college admissions readers, and the school's # of full rides offered in general). Though there are many factors, it is entirely possible to receive a full ride and merit is one of the most, if not the most, common ways to do so. Scoring in the top percentage with your high school GPA and standardized testing, coupled with well written essays that demonstrate you as an individual, will greatly improve your chances of receiving significant scholarships. There are a variety of scholarships that schools offer, with varying merit scholarship amounts depending on where an applicant falls in the applicant process, so it's best to do your best in all aspects of the application to maximize your potential for receiving scholarships.
As an example, several friends of mine qualified for an exclusive scholarship offered to 20 of the 1000 incoming freshmen that covered their full tuition and boarding for all 4 years. This was specific to the university and to this year's incoming class, but there were also a variety of other scholarships ranging from $5k-$25k a year. Hope this helps, and best of luck in the process!