Just ordered my second set from Bimmian today (they are $5 off right now, so $30 total with shipping). They hold great and like 1speed mentioned fool everyone. After more than a year I finally knocked one off with the pressure washer at the quarter-mat. Otherwise, no issues (not with fading either, but my car is garaged 95% of the time.
One sheet would certainly be enough if you are just changing the blue part of the roundel; and you will still have extras left over just in case. I was blacking out all 4 corners and ended up trimming some of the larger ones down to size to accomplish this with just one sheet - hence, no backups and the need for another set.
Do reccomend -