Should be per day, same as Ships in most (probably all) things. Do note you can only survey a COLONY.
One vehicle is enough, but it will take ages. Don't forget you can have 2! geosurvey tools on your vehicle too (as well as one on the HQ if you want). When designing the unit, very bottom left has Additional Equipment where you can select ANOTHER geosurvey equipment to double your output.
I personally only have one Unit that does ground survey, but it does it FAST, slightly twice as fast as my ships do (I start my geosurvey ships with 3 modules, and with improved sensors go down to 2 while still having 30% more points and less cost/weight). The unit is below 10k tons (which is the smallest size I use for RP purposes).