Donor area has yellow crust. Is this infected or normal? Is it true grafts are different from amount of strands that grow?

From the picture, you look to be healing well, but continued cleanings are needed to help encourage removal of the crusts. Witch hazel is an over-the-counter product that can be applied to the area during cleanings to loosen the crusts and doesn't have the potential risk of changing hair color. Good luck, and be sure to touch base  with your surgeon if you develop any concerning changes. 
Also, you are correct regarding hair follicles. The follicles transplanted have varying amounts of hair shafts, and can contain 1-4 terminal hairs. Most follicular units contain 2 or 3, but single hair shafts are common as well. The surgeon can use these different follicular units to their advantage when planning placement to achieve the most natural and dense results. 

Good afternoon and Thank you for your question, 4 days post your FUT procedure - Please be assured that the suture area looks fine and there is no infection, you will just have to wait about another 12-14 days before the suture starts falling off if its dis-solvable (or) get it removed at the clinic manually if it's non dis-solvable. 
Number of grafts may be equal or lesser than the number of strands that grow because one graft could have anywhere between 1 -4 hair strands. 
Regards,