How large does a system have to be for $\Omega$ to be the multiplicity of the most probable macrostate?

Generally speaking, how large does a system have to be for the total multiplicity of the system to be (100.000000000000000000000000001 percent of) the multiplicity of the most probable macrostate? I noticed that in Schroeder's Thermal Physics, entropy is defined to be $$S = k \ln \Omega$$, but what is meant by $$\Omega$$, the multiplicity of the system or the multiplicity of the current macrostate, isn't given, so I inferred that it doesn't matter because the system in question is assumed to be large. If I'm correct, then it makes sense to ask this question.