some use a bed of chopped greens underneath the parsley bed and some don't.
Figure 2-3 bunches of parsley per box.. Depends on how selective you are on color, freshness of the greens etc.. Better to have extra as it's cheap enough. Remember most of the middle will likely be covered with meat so don't use your best parsely under what will be covered.
Try to group them in bunches (think multiple groups of broccoli crowns).. dont try to manually place each stem in as it will take forever and get you to the same place.. just group in small batches, pack it tightly, level it out and them fill in or correct and glaring problems.
Stem length depends on what you put underneath and what meat it is and how you position.. Figure enough heigt to get up about 1/2 to 2/3 of the way to the top of the box for a single layer entry.