Apartments are rented at different nightly, weekly, and monthly rates, which vary by season and number of guests. The nightly rate for a two-bedroom in high season might be $400 for two people and $750 if you cram in six. In most cases, we're listing the low season per-person price, assuming your group fills all the beds.
When investigating possible apartments, look closely in particular at the number of bedrooms, as well as at the number of people an apartment sleeps. More often than not, "sleeps four" means a one-bedroom unit, with two guests in the bedroom and two on a fold-out couch--fine for a family, less than ideal for two couples. Ask the owner or agency about the setup if it's not mentioned in the listing.