You're confused. "Supply" refers to goods or services, not to appraisal orders. In appraising parlance the appraisals are the supply and the appraisers are the suppliers. "Demand" refers to the use and consumption of those goods and services, so in the mortgage appraising business the demand for services is coming from the lenders.
The AMCs are just purchasing agents for the lenders. They aren't actually part of the supply/demand except to the extent they are designated to act for the buyers on the demand side.
Some appraisers can and do operate completely outside of the AMC loop, thus demonstrating the point that the AMCs aren't part of the overall supply/demand equation.
my clumsy attempt at Humor