What is the rotational symmetry number used to calculate the rotational entropy from an ORCA frequency calculation? The quantum chemistry software ORCA gives the following warning on frequency calculations:

CAUTION: The rotational entropy is not quite correctly treated here
         because it includes a symmetry number that is not yet correctly
         implemented in ORCA!
For a nonlinear molecule the correct rotational entropy is:
    S(rot) = R*(ln(qrot/sn)+1.5)
    R    = 8.31441 J/mol/K = 1.987191683e-3 kcal/mol/K
    qrot = 4922043.1563766
    sn is the rotational symmetry number. We have assumed 3 here
       if it is different for your molecule then you should correct
       the printed rotational entropy by manually evaluating the equation
       as given above

For convenience we print out the resulting values for sn=1 - 12:
 sn= 1  qrot/sn= 4922043.1564 T*S(rot)=    10.02 kcal/mol T*S(tot)=    34.72 kcal/mol
 sn= 2  qrot/sn= 2461021.5782 T*S(rot)=     9.61 kcal/mol T*S(tot)=    34.31 kcal/mol
 sn= 3  qrot/sn= 1640681.0521 T*S(rot)=     9.37 kcal/mol T*S(tot)=    34.07 kcal/mol
 sn= 4  qrot/sn= 1230510.7891 T*S(rot)=     9.20 kcal/mol T*S(tot)=    33.90 kcal/mol
 sn= 5  qrot/sn=  984408.6313 T*S(rot)=     9.06 kcal/mol T*S(tot)=    33.77 kcal/mol
 sn= 6  qrot/sn=  820340.5261 T*S(rot)=     8.96 kcal/mol T*S(tot)=    33.66 kcal/mol
 sn= 7  qrot/sn=  703149.0223 T*S(rot)=     8.87 kcal/mol T*S(tot)=    33.57 kcal/mol
 sn= 8  qrot/sn=  615255.3945 T*S(rot)=     8.79 kcal/mol T*S(tot)=    33.49 kcal/mol
 sn= 9  qrot/sn=  546893.6840 T*S(rot)=     8.72 kcal/mol T*S(tot)=    33.42 kcal/mol
 sn=10  qrot/sn=  492204.3156 T*S(rot)=     8.65 kcal/mol T*S(tot)=    33.36 kcal/mol
 sn=11  qrot/sn=  447458.4688 T*S(rot)=     8.60 kcal/mol T*S(tot)=    33.30 kcal/mol
 sn=12  qrot/sn=  410170.2630 T*S(rot)=     8.55 kcal/mol T*S(tot)=    33.25 kcal/mol


Now, I have got the following two questions:


What is the rotational symmetry number $s_n$, i.e. how is it defined?
What symmetry number $s_n$ would a molecule with no symmetry, i.e. $C_1$ point group have? The way I understand it, the rotational symmetry number reflects the rotational symmetry, e.g. $\ce{CHCl3}$ would have $s_n=3$ as there are three equivalent structures superimposed by the $C_3$ symmetry element. So that my guess is that $s_n = 1$ in that case. Is that correct?
Your #2 is exactly right, as far as I know, which answers your #1.

A key thing to remember is that the rotational symmetry number doesn't include reflections, so molecules in groups like $C_s$ still have $s_n = 1$ since there are no superimposable orientations achievable only with rotations.

For reference: Table II of the NIST CCCBDB introduction to thermochemistry is a nice, concise summary of the symmetry numbers for the common point groups observed in chemistry.