The seed phrases contain a checksum in the last word.

Arbitrary combinations are possible for the first 11, 14, or 23 words, but for the last word you then only have 128, 64, or 8 possibilities that make a correct checksum.

These get less, because for longer seed phrases the checksum gets longer and less of the bits in the last word are freely chosen.

Since the 2048 words on the list can encode 11 bits – 2^11=2048. 12 words encode 132 bits in total, 15 words 165 bits, and 24 words 264 bits.

For 12 words the secret entropy is 128 bits, the checksum 4 biits, so 7 of the 11 bits of the last word are still freely chosen, which means 2^7=128 possibilities.

For 15 words, secret entropy is 160 bits, checksum 5 bits, 6 bits of the last word freely chosen, 2^6=64 possibilities.

For 24 words, secret entropy is 256 bits, checksum 8 bits, 3 bits of the last word freely chosen, 2^3=8 possibilities.

The details of the algorithm are in: https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki

To show you which seed phrases for a beginning chosen by your exactly are these possibilities, you need some tool that implements this.

Or you randomly choose not words, but directly the secret entropy and convert that back to a seed phrase, but you’d also need an inplementation of BIP-39 for that.