When Boss's Chauffeur dies, its second ability and its persist ability trigger.
Resolve the persist trigger, returning Boss's Chauffeur to the battlefield from your graveyard with at least one +1/+1 counter due to its ability.
Resolve the second Boss's Chauffeur trigger, creating a number of 1/1 Citizen creature tokens equal to the number of +1/+1 counters that were on Boss's Chauffeur when it died.
If you created a Citizen this way, activate Phyrexian Altar by sacrificing a Citizen, adding ({1} magic symbol) .
Repeat.
If you controlled at least one additional creature to start the combo, this combo will net infinite Citizen tokens, infinite +1/+1 counters on Boss's Chauffeur and infinite colored mana.