The numbers matter because a group shares its fixed costs. The guide, the vehicle, and any place given to the priest cost the same whether twelve people travel or thirty, so a smaller group pays more per person, and the step from a minibus to a full coach changes the price again. Companies draw these lines differently and no Church body or regulator publishes a standard, so treat any figure you are quoted, ours included, as that company's own, and make sure it appears in the written offer.
Ask what happens if enrollment falls short, because a price built on a large group can change a great deal when fewer people book. The usual outcomes are a new price for the smaller group, a merger with another group traveling on nearby dates, or cancellation with a refund. The written terms should say which applies, the date by which the decision is made, and exactly what is refunded. Until the group is confirmed, do not buy your own flights.
Size also matters at the shrines. The Shrine of Fátima, for example, asks organizers to register in writing with the number of pilgrims, and says that space for a group's own celebration depends on availability and can be limited by the size of the group. A parish that cannot fill a group alone often does well to join with a neighboring parish.