Well, when the puzzle was in the actual Listener magazine, entry was particularly low - sometimes in the single figures. Since it's been in The Times, and available internationally online, the average entry has obviously 'soared'. I think it now averages between 400 and 500 each week - but this obviously rises to a 4-figure number on 'easy weeks'. Every now and then a particular stinker brings this average way down!
There is a fairly new official website incidentally (www.listenercrossword.com) with loads of fascinating information - and puzzles are always discussed on the forum at www.crossword.org uk (after the solution is published, naturally!).