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

Looking at your picture the wound appears to be healing quite well.  It is not infected.  When the incision is closed with stitches, some fluid can ooze out of the wound.  This will then dry out and form a "crust."  It is a normal part of the healing process.  The scabbing or crust will flake off on its own.
Each "graft" is actually a "follicular unit."  In a normal follicular unit the number of hairs ranges from one to four.  Thus for a specific amount of follicular units transplanted, you will have many more number of hairs.

Yellow exudate that dries is normal. You should treat it with hydrogen peroxide diluted with 1 part water 1 part peroxide and clean it!

First, don't worry about the donor site.  It looks fine.  Sometimes these areas can ooze a bit because of the injury, but this one is clearly not infected and looks like it was closed expertly.Second, each graft is a "Follicular Unit" which generally can have from 1-4 hair follicles within it.  Therefore, the number of hairs that grow can be many more than the number of grafts.  This is why some facilities that charge by the graft may not really be offering more hair, they just divide the grafts when necessary to make it appear that they have done more work.Good Luck!

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,

The presence of a yellow exudate on the recipient area is normal and this should be washed off before it forms scabs that stay around for quite a while