Misa,
Ideally the SS7 signalling links dimensioning is done based on the message load. Meaning that you need to first findout what type of messages are being sent on the SS7 links. For each type of message, you need to know the messages length (Bytes) and what is the frequency of each type of message (that is generally dependent upon the BHCA). Once you know all this you only need to do the maths for bytes per second. If you have E1 then each link will be 64Kbps and if T1 then it will be 56Kbps. Devide the Bytes (hence K Bits)per second requirement with the link speed and multiply with 40%, and you have the total number of links.
If you are using the SS7 links for ISUP, a general rule of thumb is to have 40E1s, per pair of SS7 links.