function pack(...)
return arg
end

types = pack(GetObjectCreaturesTypes('garrisonName'))
army = {}
for n, id in types do
  if id > 0 then
    army[length(army)+1] = {id, GetObjectCreatures(id)}
  end
end

-->>>army == {{id, number}, {id, number}, etc.}

You can use any algorithm you like for working with the parameters; this is just an example. GetObjectCreaturesTypes returns the types (IDs) of creatures (and zero if it's not a complete set), and GetObjectCreatures(id) retrieves the number of creatures in the object for a given ID.