Babis,
all depends on your Average Call Holding Time(usually in seconds)… after knowing that you just multiply times the 20,000 BHCAs and that would be the total traffic (in seconds). If you want that on Erlangs you just divide by 3600 (60 times 60, first to convert to minutes then to erlangs)….
Traffic(Erl)=BHCA*ACHT(sec)/3600