there is no maximum number of missing characters that you can recover 1 to 3 missing characters can be easily recoverable but above 3 it takes a few months or years.
Are you confusing WIFs with mnemonics?
In a mnemonic more than 3 missing words starts becoming very hard to recover but in a Base58 WIF it is still very easy to recover keys missing 5-6 characters and anything higher than that will start taking hours and exponentially become harder.