If you have enough bandwidth, the calls will fit on the SIP Trunk IP pipe.
Be careful of the 6 concurrent calls allowed into CUE at any given time limit.
This would include things like Auto Attendant & Voice Mail so if calls are coming into the extensions via AA, could be a problem.
UC560 does 12 concurrent BTW and starts at 16 users. Also has bigger and expandable CUE Flash (up to 8G so 4x that of UC540).
Steve