Using "trunk" in the sense of WAN connections acting individually or in concert, the USG 50 only has two. I think the WANs act as one trunk if both connections are used together in some dual-WAN sharing scheme.
Another meaning of "trunk" is an interface that passes both VLAN tagged and untagged messages. With VLANs, one or more LAN ports connected to smart switches can be trunks in this sense.
It will prove useful to obtain both the firmware version 3 User Guide and the version 2.21 User Guide, plus various groups of Notes that ZyXEL has published. When you have digested the 1400 pages, more or less, you too can be an expert.
In particular, read page 222 of the 2.21 guide. (ZyWALL USG 50_2.21_Ed4.pdf)