From the text:
At 5th, 10th, 15th, and 20th levels, the bonuses on weapon attack rolls, damage rolls, and skill checks and to slayer DCs against a studied target increase by 1. In addition, at each such interval, the slayer is able to maintain these bonuses against an additional studied target at the same time. The slayer may discard this connection to a studied target as a free action, allowing him to study another target in its place.
So at level 15, I can have four studied targets at a time. Can I study them all as a swift action, or do I have to use an action to study each one individually?
First instinct is that they all use the same action, but I don't actually know.
Pretty sure you use one Swift action per target.
to study is just one person at a time per action. So you'd need to use 4 actions. Fastest is 2 swifts and a move and a standard used as a move.
really though since it's a swift you don't need to do it before you're attacking that person. And the extra uses can be done on NPCs that you are suspicious will show up later as enemies.