While the numbers of containers lost each year varies it is certainly in the thousands. Thankfully most sink. Others wash ashore like our Frito container here on the outer banks which remained quite tasty! They are a risk day or night but thankfully we don't hear much about sinkings being rampant as there is just too much ocean and too few FLOATING containers at any point in time to be of much concern.
BTW...a good radar will paint a small can buoy so if a decent portion of the metal container is above water...you will see it on radar if not with your eyes.
Most shipping containers used nowadays have the annoying habit of floating either 1' or so exposed or 1' or so under the surface (depending entirely upon what's inside - be it stuffed toys or refrigerators).
It is a SOLAS requirement that shipping containers have one or more sacrificial plates fitted that corrode quickly in seawater and are *supposed to* sink the container within 24hrs or so, but you're only going to be concerned about hitting a container if:
a) You're in a shipping lane (so you should be keeping a good watch anyways) and
b) There's been a storm in that area in the last 24hrs that *might* have dumped a container or two off the high side of a rusted hulk (oops! sorry.. I meant "ship").
In any case, given that the sea is never flat immediately after a storm on any of the world's oceans, practically speaking, you ain't going to see it - even on radar - unless you've got the wave return setting down really low and even then only if you get lucky.
Summary: If you're worried about hitting a container, stay out of the sea lanes - but then if you feel safer knowing that there are ships around to rescue you if you get sunk, stay close to the sea lanes. Of course, you can't go too far from the equator because you might get hit by a whale and you don't want to be close to land because you might find yourself on a lee shore..
..so, in the end, if you're worried about hitting something - anything at all - stay home in bed, leave the boat in the dock... and leave the oceans to Simon.