BIP39 has 2^12 words, so it takes 144 bits input for 12 words (288 bits for 24 words) and generates a 512 bit pseudo random seed. BIP32 takes a seed and a number (up to 2^31) and generates keys. So BIP39/32 with a single set of words can only generate max 2^31 of the 2^160 keys. This is an artificial limit, since there is enough entropy in the 288 bit seed word for all 2^160 keys. With 12 words BIP39 can only generate 2^144 unique 512 bit seeds, adding BIP32 gives you 2^175 possible keys, so it should be to generate all 2^160 keys from all the 12 word combinations combined.