There are no general rules for tango as to whether you have to dance with someone at all. But I think in every big city there is definitely at least one larger, nationally known milonga in which panmixia is practiced. All other milongas, practilongas and socials are special biotopes with their own environmental conditions and rules. In some only the dance partners dance together, in others the couples. In some socials, invitation only occurs within the cohort. I myself only dance with 5 women, although we have a big scene in Hamburg.