Hi Nagra,
The approximate relation is 4:1. It means, if there are 64 Abis links in a BSC then it is recommended to have atleast 16 A links b/w BSC& MSC. This is due to the mux feature in Abis in which 4 subscribers share a single path. But in BSC/TRC with MSC, the A link allows each subscriber with a slot. Hope this clears. Thanks