Skin and hair can shed after the procedure starting few days after surgery and it can be normal. However, there is also possibility (if something went wrong) that there is necrosis of the scalp skin. You should check immediately with you surgeon
What can be done depends on the quality of your donor site and the size of the defect that you want to cover it is quite hard to discuss it without seeing your pics. I do a lot of secondary hair transplants both after FUE and FUT procedures . They are usually successful in some cases we do body hair transplants (if scalp donor is not available)
Being only 27 years old you can expect more hairloss in your future. First of all I would strongly recommend FUE method to avoid ugly linear scar that would prevent you from wearing short haircut EVER in your life. I prefer ARTAS robotic surgery since it is incredibly fast and precise but also offers protection of the existing hair. But mind that ARTAS is just a machine and has to be run by excellent surgeon and team (just like in racing you have to have a great car but also superb driver and the team to win)
As for the number of grafts depending on your desired hairline you would need 1500-2000 grafts