On domestic (including short international) flights it is almost always the minimum. It's the same for all airlines. Occasionally, longer domestic flights will carry an extra. Crew scheduling will often schedule a deadheading F/A as an Extra instead of a DH. The 737s have two extra F/A jumpseats; the A319/320 has one. Don't know about the 321.

International flights (long haul) likely have extras. I don't fly a widebody fleet so don't know the specifics.