My understanding is that waypoints begin with "GC" for geocache. Early caches had 1 digit after this, then 2, 3 and now 4. I have found caches from 2001 with only 3 digits after the GC.
If that is the case, then with 4 digits there are a total of 1,727,604 caches that can be added. This would include caches that were submitted, but rejected and archived caches.
My guess is that we will move to "GC" plus a 5 digit suffix soon, allowing 62,193,780 total caches. Then extended again to 6 digits allowing 2,238,976,116 caches. This will then be the limit that some GPSr's can handle for waypoint names - but this is some way off.
When the length is increased to 7 digits, every possible spot on earth (subject to .1 mile restriction) will have a cache, plus 3 old, archived ones.