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Generally, RB is calculated by:
RB = (Vcc x hFE) ÷ (5 x IL)
where:
Vcc is the transistor's power supply (12V)
hFE is the transistor's gain (check your exact transistor(A,B,C) and datasheet, but let's say 120)
IL is the load current - see below.
You've missed out a critical parameter - the relay coil's resistance. With that, we can calculate the load current (I = V / R) then use the above to get the resistor value.
P.s. Don't cross post your questions to other forums.