26 out of 24 words? Are you saying you have two extra words?
If that's the case and since you say you know the 6 first words in the correct order, there should be 1.2 *10
18 possible combinations. Assuming your seed phrase is BIP39, around 4.7 * 10
15 out of those combinations should be valid on average. That's a very big number and you can't brute-force your seed phrase.
So it's 20x19x18x...x1 = 20! = 2,432,902,008,176,640,000
If I got OP correctly, there should be (20!)/2 possible combinations.
(Seed phrases with invalid checksum are also included.)