As a matter of practice, I need to know if it is a good idea to include the APN/Parcel number when ordering a flood determination. I am trying to convince others that this is a good practice, but I am meeting some resistance.

You need to provide what your vendor requires to identify the property. Why go further than that?

Well, here's what happened and providing the APN probably would not have made a difference anyway.

We had an apartment complex with multiple buildings at one address and two parcels. My understanding is that we need two flood determinations; one for each parcel.

I'm just trying to figure out a good internal control to make sure this doesn't happen again.

Also, where in the flood regulations does it state I need two determinations?

Multiple parcels are not the issue. Multiple buildings are. See the reverse side and instructions for the SFHDF:

MULTIPLE BUILDINGS: If the loan collateral includes more than one building, a schedule for the additional buildings/mobile homes indicating the determination for each may be attached. Otherwise, a separate form must be completed for each building or mobile home. Any attachments should be noted in the comment section. A separate flood insurance policy is required for each building or mobile home.