The way I look at it is like this:
Given that sharks & rays are constantly losing and replacing teeth, it's possible to speculate that a single animal may produce somewhere in the neighborhood of 10,000 to 20,000 (or more, as I've seen estimates closer to 50,000 -- but I'm going with a more conservative number for sake of example) teeth in its lifetime. Figure that there were, in prehistoric times, probably quite a large number of sharks each year out there. Figure that all those sharks were losing and regrowing thousands upon thousands of teeth. Then figure that we're talking about a period of millions of years in which these fossil deposits come from. That's a lot of sharks!
Even if only a small percentage of those teeth were actually fossilized, you're still talking about an /awful lot of teeth/! While there are, of course, a finite number of teeth fossilized out there... I speculate that fossil collectors haven't even put a dent in the total number. Perhaps the upper most layers of sand and sediment may be a bit thinner on teeth than before.. or even covered up by those renourishment projects.. but given time and eroding wave action, I'd imagine further deposits will eventually come to the surface.
But that's simply my inexpert opinion. =)