When Boss's Chauffeur dies, its second ability and Enduring Renewal trigger.
Resolve the Boss's Chauffeur trigger, creating at least four 1/1 Citizen creature tokens.
Resolve the Enduring Renewal trigger, returning Boss's Chauffeur to your hand from your graveyard.
Activate Phyrexian Altar four times by sacrificing four Citizens, adding ({4} magic symbol) .
Repeat.
If you control four or more creatures to start the combo, this combo will net infinite Citizen tokens, infinite +1/+1 counters on Boss's Chauffeur and infinite colored mana.