The next one (in a few days) is SN15, which is a major upgrade with "hundreds of design improvements across structures, avionics/software & engine.", according

The next one (in a few days) is SN15, which is a major upgrade with "hundreds of design improvements across structures, avionics/software & engine.", according to Elon. "Next major technology rev is at SN20. Those ships will be orbit-capable with heat shield & stage separation system. Ascent success probability is high. However, SN20+ vehicles will probably need many flight attempts to survive Mach 25 entry heating & land intact." Surviving the orbital entry and landing is probably the most difficult challenge yet. After that the next major challenge is landing on Mars.