There is no shortage of numbers and, as Phil says, they will definitely not be recycled.
There are over 10,000 "9" numbers left
A large number of "8" numbers have not been issued yet (they are used for allocations for ships which are not newbuildings)
I suspect that there are a good number of never issued "5,6,7" numbers
I think that only a few "3" or "4" numbers have been utilised
As far as I know "1" numbers are at present mainly used for yachts; also some of these and some "2" numbers have been used for small vessels outside the IMO system and only appear if that vessel subsequently applies for an IMO number (eg http://www.shipspotting.com/gallery/photo_search.php?imo=2838358
Some "invisible numbers" have been used for database purposes as temporary internal IDs
And of course, though there may be technical reasons not to use them, no "0"(zero) numbers have been used, that's another 99,999 more.