To do this you need to define the Tracks in the Composition to repeat via a Custom Property. Then specify the values for those Custom Properties on the scommand command line call. This should allow you to control the number of virtual users for each Track from scommand.
Open the composition you want to run. Inside of the composition editor, open the minimized Properties tab on the bottom. From here, create a custom track property:
Then on the track repeat settings, enable repeat, and set repeat by property:
Double-click on [not specific] and select the custom track property we created from the Property Chooser:
Then after saving the composition you will want to launch the Composition from scommand and add the custom property value:
scommand cmd=play name="/Load Tests/Comp 1" username=bob \
password=secret url=http://<CloudTest URL>/concerto \
customproperty="Band 1/Track 1/track1VUcount=150