How I do to manage so many Overlords?
Aside from general scouting (which is something you should do constantly), a couple of practical examples come to mind:
On most maps you can position two overlords around the base of your opponent (on some maps or spawning positions this only works with one overlord due to lack of air space behind bases). Put two of the first couple of overlords there and fly in from time to time to scout for early game things your opponent is doing ("two-port banshees. oh sh...").
One use of Overlords is to spread creep on empty expansions near or around your opponents half of the map. This prevents him from taking rogue expansions and annoys him by requiring him to send a marine / stalker / whatever to the location of your Overlord and drive him away. Of course, this also provides you with information on the economic status of your opponent.
Scatter the overlords behind your bases where Medivacs are likely to fly by if your opponent wants to drop in one of your bases. This enables you to detect the drop early and prevent it from even happening if you're fast enough.
If you want to use them as "warning"-sentries of some sort, you can position them just outside the edge of cliffs. This way, when your opponent attacks them, they immediately automatically fly up the cliff without being killed (if your opponent has no air units) and also cause the advisor voice to say "We're under attack" (or whatever).
This may seem subtle, but it is actually very important: The creep connection between your main base and your natural. If you reach Lair and haven't managed to connect the two bases with creep tumours yet, you should at least have overlords positioned along the way to start spewing creep as soon as you reach Lair. The few seconds more it takes a couple of zerglings to reinforce your natural to a Terran push can make or break a game.
Morphing Overlords to Overseers will often times save your game. I can say this from actual experience. Countless times I have been saved from Dark Templars last second because of morphing Overseers.
Even if you spread creep constantly, it will often happen that the tumors just get destroyed during battles and you have no time to build them again. In some cases you will try to finish off your opponent. In those cases, use overlords to build a "creep-street" along the path that your reinforcements walk.
And finally:
Most of the time you will still have a ton of overlords which (who?) just sit around your base doing nothing. Take a bunch, put them on a hotkey and take them into battle. If you send them towards your enemy just a couple of moments before the rest of your army arrives, the Overlords will usually eat up the first volly of attacks of some units, which can be useful against Thors for example (Thors lose a lot of DPS with that initial missed attack).
And always remember to research the speed upgrade for Overlords on Lair :)
Like any strategy, it really depends on what your opponent is doing.
Scouting is probably the globally useful strategy for them. Unless your opponent is prime to build anti-air units just to shoot down your overlords (which isn't a bad strategy), you can gain so much intel by placing overlords all over the map. Try to hide them from ground line of sight by using cliffs and doodads.
I would only recommend using their creep generating abilities to connect expansions. Placing them ahead of your army and just letting them sit there is prime sniping targets.
Only build detectors as needed. This is where scouting comes into play. If your paranoid about cloakies or roaches, just build one at a choke point.
Scattering doesn't sound like a good idea. If you want to protect them, clump them up around a bunch of queens.
So by default, place them around the map. If your opponent is air sniping them, bring them back under the protection of your queens.