Single engine approach and landing is flaps 20.
As a general rule of thumb, on all the CRJ-family, if a failure case directs you to a flap selection other than 45, usually flaps 20 is the value.
As far as I know, the only case where you would land at flaps 30 is when the flaps will not move from that position i.e. the "flaps fail" and "slats and flaps fail" cases.