How do I calculate the number of guards needed for a site?

Short answer

Determine the number of simultaneous posts needed (e.g. 1 at the entrance, 1 at the warehouse).
Determine the coverage schedule — 24/7, day-only, night-only.
Calculate total coverage hours per week, per post.
Divide by the legal working hours per guard, respecting mandatory rest between shifts.
Add a 15-20% margin of backup guards for leave and absences.

Labour Code, Art. 112-114 — the standard working time (8 hours/day, 40 hours/week) as the basis for the calculation.
Labour Code, Art. 135-137 — minimum daily and weekly rest, which limits how many consecutive shifts a guard can cover.

Example: a 24/7 post means 168 hours of coverage per week. With 12-hour shifts, that's 14 weekly shifts — respecting legal rest, that needs at least 4-5 rotating guards per post, plus 1 backup guard for leave.

The calculation has no backup margin, and the first sick leave or absence leaves the post uncovered.
Legal rest hours aren't accounted for, leading to non-permitted consecutive shifts.
The estimate is done once, at contract signing, with no recalculation when the schedule changes.

You record the number of posts required per interval on each site, and the coverage report shows where the roster has fewer guards than the posts required. The calculation of the number of guards (the steps above) is yours to do; 4guards does not compute it.
From the Plus plan, saving a shift checks overlaps, daily rest (12 hours by default) and maximum shift length, while more than 48 hours worked in a rolling 7-day window only raises a warning; weekly rest is not checked. The save can be blocked or only warned about, depending on the account settings. It is a planning aid, not a legal opinion.
Guard records with the attestation number and an optional deadline, as a basis for sizing the backup pool; 4guards does not suggest backup guards.