Speaking at the 2020 #HumansToMars virtual conference, @elonmusk says that SpaceX will begin building the first Super Heavy booster (SN1) "this week." Engine count will be reduced to 28 "to simplify the configuration." Outer ring of 20 engines will use 250 ton-thrust Raptors.
@elonmusk: "It's within the realm of possibility" that Raptor's thrust-optimized variant can achieve a thrust-to-weight ratio of 200:1. AFAIK, that would be even better than Merlin 1D, which holds the current world record of ~180:1.
And Musk reiterates that Raptor's ultimate vacuum ISP target is still 380s, close to the physical limit for the O/F ratio Raptor operates at.
