“More than” is preferable to “over” when referring to numbers of people, places or things:
More than 500 people attended the national conference, representing more than 50 local chapters.
Reserve “over” for monetary amounts and ages.
The charity walk raised over $10,000 to benefit Senior Services, which helps needy individuals over 65.
Rule of thumb: If the question is “how many?” use “more than.” If the question is “how much,” use “over.”