Just three signatures are required.
The Private Ways Statute, Section 3101, states: "...the owners of any 3 or more of the parcels, as long as at least 3 of the parcels are owned by different persons, may make written application to a notary public to call a meeting."
The wording does seem to imply that all owners of a given parcel must sign. On our notary application to call of meeting, however, our attorney required only one owner signature from each of three parcels owned by different persons. I think of this as similar to "one vote per parcel". Any owner from a parcel may speak (vote, sign) for that parcel.