Multi strand metallo-supramolecular polymers are self assembled by pyridyl-functionalized porphyrin derivatives on a Au(111) surface through pyridyl-Cu-pyridyl coordination. Single-molecule-resolved characterization by scanning tunneling microscopy reveals a novel chain growth polymerization mechanism for multi strand supramolecular polymers. Furthermore, by varying the growth temperature and adding specific molecular modulators, both the length and the width of the polymers can be controlled.