I'm going to go with $4,150

Your questions is a little to vague to be answerable.  Are you asking about whether a student is a qualifying child for the credit for other dependents?  In which case there is no limit, they must not provide over half of their own support.  Or is the student over age 24, or not full time and are you asking if they are a qualifying relative for the credit for other dependents, in which case the gross income test would be applicable and the amount is $4,150 for 2018.

This is all laid out in Notice 18-70

https://www.irs.gov/pub/irs-drop/n-18-70.pdf