Save-the-dates go out earlier than almost anything else on a wedding timeline, which is exactly what makes the number hard to pin down — you're ordering before the guest list has finished changing shape.
A save-the-date goes to a household, not a person. A couple who's invited as a pair gets one card, not two. Count your guest list by household before you count it by name, and the number drops fast — a list of 150 people is often closer to 90 households once couples and families are grouped.
Three things eat into a save-the-date order that a straight headcount doesn't account for:
The standard cushion is 10–15% over your household count. For 90 households, that's 100–105 cards — not a large jump in cost, since the difference is a handful of sheets, not a second printing job.
Reprinting later almost always costs more per card than the original run did, because you're paying for a second setup rather than a larger single batch — true whether you're printing at home or through a shop. Ordering a few extra up front is the cheaper mistake to make.
Not every guest on the final invitation list needs one. Save-the-dates typically go only to guests you're certain about at that early stage — the people you'd invite in any version of the wedding. A guest you're still deciding about can be added to the full invitation later without ever having received a save-the-date; nothing about skipping this step is impolite.
Once the household count is settled, Promuse exports the save-the-date, invitation, and RSVP from the same design — so a number you land on here carries straight through to what actually gets printed.