Its not simultaneous calls. Its capacity of a 4+4+4 cell taking 25merl per sub.
25merl: This is the average erlang that shall be utilized by a subscriber. This value is used for all theorotical calculation for capacity issues.
Calculation: 4TRX supports a maximum of 18.4erl. 18.4/.025 = 736 subscribers. The same for all the 3 sectors gives you around 2200 subscribers. BUT NOT SIMULTANEOUS.