For bluebirds as for as predators - raccoons, snakes, and probably cats are the biggest problem. I put my boxes on post where I installed a baffle, but not sure if you can find one. You might could make a baffle out of sheet metal or an old stove pipe. I occasionally get wens taking over the box. The hole is too small for starlings. If house sparrows start to build a nest in the box, generally the male will put a few sticks in the box then set on the top singing to attract a female. I place a small mouse trap in the box and it catches his leg, but can't get the trap through the hole. I open the box and release it. For some reason it decides this isn't a friendly neighborhood.
As for the number of boxes - I would say 3 or 4, but don't put the too close together because they will fight.
I placed mine about 5 feet off the ground and if possible not facing the south and west because of the evening sun shining in the hole. I either drill a 1 1/2" hole or overlapping 1 3/8 in holes - that is what I did with the latest batch. I also make it so one side pivots so I can clean the box. I generally wait until around Thanksgiving to clean the box out. I can make two boxes out of a 1 by 6 by 10' board. These are made of poplar. I leave the inside unfinished, but paint the outside to make them last longer.
I do not have plans. Like I do most of my stuff, I wing it. Malcolm / Kentucky USA
Oh, on the side that pivots, that side has to have clearance at the top for it to swing open, have to have a place at the bottom for you to get you fingers on it to open and it has to be loose enough for it not to bind.