Guest count comes first
Guest count decides which yacht is even offered, because motor-yachts carry hard capacity limits set by their safety certification. A 24m day yacht might be certified for 12 guests cruising; a larger vessel 20 or more.
Say "around 10" and arrive with 15, and the crew cannot legally take the extra five — so give the real number, including children and infants. Pricing is per yacht, so it sizes the vessel, not the bill.