How many credit hours i have to take to be considered as a full time student

[MinhT]

The IRS defines a full student as follows:

A full-time student is one enrolled for some part of five calendar months (whether or not consecutive) in a given year for the number of hours or courses considered full-time attendance by the school that the student is attending. 

You'll need to ask your school the number of credit hours needed to qualify you as a full-time student.