I haven't been keeping them so the true number is lost forever.
So I will simply say: A staggering number.
Let's do some Fermi estimation!
10 maps per gameday, let's say 5 are new each time.
3 new maps per impromptu
Gamedays have been around once per week since 2008, I've played in many since then. There's been about 300 weeks since 2008, Let's say I played in a third of those (averaging out my peak times and inactivities) so 100 gamedays. 500 maps from gamedays
Impromptus have run about 3 per week since maybe 2011 maybe I've played a sixth of those. Let's say 200 weeks since then, making for 600 imps and 100 I've played in. 300 maps from impromptus.
Then 20 for each contest TF2 maps has run, which I estimate to be around the 20 mark. So another 400.
1200 and that's just the maps made here at TF2maps.net I've probably played at least another 200-300 on other servers that have nothing to do with TF2maps.net
Puts the total for me at around 1500. Which is similar to Boojum's total so I'm guessing my estimations are pretty accurate