There is no upper limit on number of atoms and basis functions, you can use Multiwfn to analyze arbitrarily large system as long as your available physical memory is adequte.

Multiwfn itself does not employ RI to accelerate any analysis, however you can enable RI in ORCA to reduce cost in the process of quantum chemistry calculation. RI approximation should not detectably affect quality of resulting wavefunction and thus analysis result of Multiwfn.

Best regards,

Tian

I've looked through the manual and found no information on either topic.

Thanks.

Jim Kress