pmacct does not come with an upstream systemd service file. Since upstream lacks any startup scripts, I assume it is expected that downstream adds these themselves.
Here is my current service file which seems to work for a single database config. Obviously the wants/after should probably go into the override.conf on a system that uses the database.
The file needs to be invoked the (a symlink) firstname.lastname@example.org directive as you cannot listen on multiple interfaces afaik.
ExecStart=/usr/sbin/pmacctd $OPTS -f /etc/pmacctd/pmacctd.conf -i %I
p.s. the wants/after could potentially be expanded with sys-subsystem-net-devices-%I.device though I currently have the -w in my $OPTS to hopefully cover that ...