When setting up a SIP trunk on a ip office essential, where do you tell it the channels? in the basic version it's an option when you set up a new trunk? I'm I over thinking or just not seeing it? we have 15 sip channel licenses

When you create the URI on the line, under Call Details tab, you specify Max Sessions.

I create 2 separate URI's for each SIP trunk, one to control outbound CID via the SIP tab on the User, the other to control CID via the ARS table like you would for a PRI. If I have 15 SIP trunk licenses, I will use 15 Max Sessions for both URI's, and in SSA it will appear that the trunk has 30 channels. This doesn't matter, because the 16th simultaneous call will encounter all circuits busy on inbound or waiting for line on outbound.