Requirements vary by county and parcel, however, it’s typically between 5-15 hives for most properties.
A hive is most commonly defined as a colony with a queen. In most cases this means each “box of bees”, however, there are set ups where boxes have two colonies, etc. inside them so you’ll want to work with your county if you plan to use a unique apiary set up and make sure they understand what you’re doing (and that they’re okay with it)