Here's the past... So imagine how much they're growing....and growing.
(Touhou also has their own convention called Reitaisai. And from what they said this year, they had to move the event to the same building Comiket is held because of how large it's becoming.)
Quote
Comiket, Date Touhou Circles
65（2003/12/30） 7
66（2004/08/13） 50
67（2004/12/30） 98
68（2005/08/14） 229
69（2005/12/30） 232
70（2006/08/13） 366
71（2006/12/31） 574
72（2007/08/19） 558
73（2007/12/31） 793
74（2008/08/15） 885