It depends on how they are doing it. Sometimes, they will only have one pin (host pin of the month) and sometimes they will have many pins to choose from. No matter what, the host will always have enough to go through everyone. The last pin trading event offered pins such as Grand Cali, the ATIS pins (except eye, that was sometimes a bonus), Haunted Mansion, etc. They will not be trading inferno.
Also, you will have to give up one of your pins to get one from the host. Sometimes they will take just any pin. Except Phinny. If it's her, you will have to give her a weekend special pin or previous host pin. Hula and Lily are sometimes more lax on what pins they will accept.