This article explains the expected behavior of the Image Multicasting "Threshold" setting in Deployment Solution.
Deployment Solution 8.x
The value is set in the SMP Console under Settings > Deployment > Image Multicasting for "Threshold" controls when the image is multicasted vs unicasted on Deployment as seen here:
The default "Threshold" value is 10 which means multicasting is used when pushing the image to 11 or more machines. This value can be set lower if a higher threshold is causing bandwidth issues or network saturation. Multicasting deploys the image once in a broadcast whereas unicast pushes the image individually to each machine.