sir i have got a seat in special round. If i reject it, will i be eligible for csab supernumary

No, please don't think about doing this. You will be out of the counselling procedure if you try to do this. The rules are strict and straight forward regarding this.

Mentioned below are certain parameters on the basis of which candidates will not be considered eligible for CSAB UT 2019:

Candidates who have been allotted a seat in JoSAA counselling round and they reported to the participating institute within the time period.

Those who have been allotted a seat in Special Vacant Seat Filling Round, irrespective of whether they have reported to the participating institute or not.

Also visit this page for more clarity. https://engineering.careers360.com/articles/csab
If you plan to take a drop, do register for CTSE. Link: https://learn.careers360.com/careers360-talent-search-exam?icn=ctse_scholarship_lp&ici=navigation-header_ctse