You require a no. of parameters like Average Holding Time , Traffic per subsriber etc for this calculation. Here is an example taking some assumptions You may do exact calculations as U will be knowing your parameters.
Traffic for 1000 Subs with 0.1E* per subscriber is 100E
BHCA= (100x 3600 sec )/60* Sec (Avg Holding Time of a cct)
Now you need to calculate Traffic on Your DTMF resources from this BHCA
Traffic on DTMF resources= (6000 x 6* Sec)/3600 Sec = 10 E
DTMF circuits required for 10 E Traffic with required GOS from Erlang B Table is 20.
No. of cards required = 20 / No. of DTMF generators per card
* Assumptions made for example, You mayput your own figures.