Pile / St Saviour is the default starting point.
The entrance beside St Saviour’s Church, opposite the Big Onofrio Fountain, is the main and most obvious starting point for visitors entering the Old Town through Pile Gate. It also puts you close to the western fortifications and the views toward Lovrijenac and Bokar.
St Luke works well from the eastern side.
The eastern entrance is next to St Luke’s Church, opposite the Dominican Church. If you are staying around Ploče or approaching the Old Town from the east, it can save you from crossing the centre only to climb back onto the Walls.
St John is the Old Port option.
The third access point is next to the Maritime Museum at St John’s Fortress. It makes sense if your day is already focused on the Old Port, Porporela or the eastern harbour side.
Choose for logistics, not internet folklore.
The circuit is the attraction. Starting at a different entrance changes the order of what you see, not the fact that the major fortifications belong to the same walk. Pick the entrance that reduces unnecessary walking before and after the circuit.
Quick answers
How many entrances do Dubrovnik City Walls have?
The Dubrovnik Tourist Board lists three: St Saviour near Pile, St Luke in the east, and the entrance by the Maritime Museum at St John’s Fortress.
What is the main entrance?
The main visitor entrance is beside St Saviour’s Church near Pile Gate and the Big Onofrio Fountain.
Which entrance is closest to the Old Port?
The St John / Maritime Museum entrance is the practical choice for the Old Port side.
References: Dubrovnik Tourist Board: City Walls and Fortresses