Documenting Similar Regularly Occupied Spaces

For spaces that are the same as you noted, you can you the same plan, elevation, and sections to document compliance. 

Is there a specific threshold to consider a space as "regularly occupied". Our building will be pretty much empty all year round

supporting some operational functions just 40 days a year on a regular 8h a day schedule.  I wonder how this impact the LEED V4 score

to get the building to be LEED Silver

There are many building types with fluctuating operating hours/schedules. Any space that meets the definition of “regularly occupied” should be included in EQ credits, even if it is only regularly occupied for a small percentage of the building operating year, as is often the case for event spaces, etc.

Be familiar with the reference guide EQ overview, specifically Regularly versus nonregularly occupied spaces, "Occupied spaces are further classified as regularly occupied or nonregularly occupied, based on the duration of the occupancy. Regularly occupied spaces are enclosed areas where people normally spend time, defined as more than one hour of continuous occupancy per person per day, on average; the occupants may be seated or standing as they work, study, or perform other activities. For spaces that are not used daily, the classification should be based on the time a typical occupant spends in the space when it is in use. For example, a computer workstation may be largely vacant throughout the month, but when it is occupied, a worker spends one to five hours there. It would then be considered regularly occupied because that length of time is sufficient to affect the person’s well-being, and he or she would have an expectation of thermal comfort and control over the environment."