In theory, one N7 channel can take care of 256 A=interface channels (256 subscribers).
In reality, you must use a big safety margin to avoid any type of congestion (can only use up to 40% of capacity). Therefore, the rule is :
1 N7 for 100 A channels (100 calls)
Easy to remember, isn’t it ?
Rule of thumb : 4 PCM (max. 120 calls) for 1 N7