Each word encodes 11 bits of data. As hosseinimr93 has pointed out, for a 24 word seed phrase the checksum is 8 bits. This means the final word has 3 bits which are not checksum, which gives 2³ = 8 possible combinations. For each of these 8 combinations, there will be exactly one correct checksum, meaning there will be 8 valid final words for any given 23 words.

For a 12 word phrase which has 4 bits of checksum, there will be 2⁷ = 128 possible valid final words.

And of course, I have to ask, why are you manually picking words to create a seed phrase? Such a process leaves you with a very insecure seed phrase and liable to have all your coins stolen.