We are extending a loan on a large ranch (1000 acres). The loan is a HELOC for 13% total LVT. The ranch has close to one dozen structures on the entire property. Do we need a flood determination on all structures or the primary residence? It is all one property address.

You would get one flood cert per TMS/parcel number. Then you'd have to go a step further and identify the number of structures that are located on each TMS/parcel number. The flood determination instructions include info on multiple structures plus your vendor will be able to tell you how to submit the info to them to ensure that it is handled correctly.