I'm by no means a great SC player, but I like to have an observer near all of my enemies' bases for obvious reasons and one near my own base or with a group of units. I'm not sure if this is too many or too little but it's been okay for me. It's invaluable to have a detector with you because cloaked/burrowed stuff can really suck.
As for management, say I have 3 observers. I will bind them to 7, 8, and 9 on my keyboard so I can easily find and move them. In case you don't know how to bind, select the unit or group of units, hold CTRL and hit a number. I usually have my air as 1, various ground units as 2-4, and 5, 6 for specific units that I want to single out like sentries for their shield or templars for their storm.
But maybe this isn't how it's done. Maybe I'm making more work for myself, I'm not sure. I just know that clicking and dragging to select units to manage can be difficult when in the midst of battle.