Is it possible to calculate (theoretically) , how many common addresses could derive from the seeds?
If there are 2048^12 different seed combinations (in 12-word BIP39) and each has 2^256 keys then we have 6.3X10^116 different addresses.
We have altogether in Bitcoin 2^160=1,46x10^48 different addresses, which is a smaller number than the first one.
How many of them are common?
A BIP-39 12-word phrase can represent 2
128 seeds (it contains a checksum). As noted above, an HD wallet can normally generate 2
65 private keys from a seed. Thus, the 12-word phrase can generate 2
193 private keys, which is 2
33 times the 2
160 possible addresses.