It is a multi-process app I believe? If so, you can fly scale count <proc-name>=<count> the process, or fly scale vm shared-cpu-8x --group <proc-name> -a <app-name> it (see: 1, 2).
Btw, autoscaling (which is a bit rough) and regular scaling are two different things (docs), and I don’t think (I may be wrong) one can autoscale one set of processes but not the others, of a multi-process app… You may instead consider using Machines (ref).