500
"People who don't practice grammar make mistakes in their writing, which causes them to look uneducated and can cost them a job down the line."
Explain the difference between the two relative clauses in the sentence.
What is "The essential clause tells the kind of people who make mistakes: those WHO DON'T PRACTICE GRAMMAR; the sentence requires the clause to make sense.
The non essential clause adds info to the sentence--the result of making common mistakes--but this could be omitted without ruining the sentence.