The Reitaisai Committee has released the list of all registered circles for Reitaisai 11. There are 4317 circles in total.
Previously, from 2011's Reitaisai 8 onward, the circles in Reitaisai had plateaued at around 5000. See
here for the detailed numbers (that's the Chinese Touhou community's new wiki). My source once told me the Reitaisai Committee was planning to do something to make the huge number more manageable, such as extending Reitaisai to two days. Seems there's no need for that this year.
As always, Team Shanghai Alice is attending. So is Twilight Frontier.
Historical note:
In case you were not around the community in May 2012, for Reitaisai 9 - the day of ZUN's wedding, someone else managed the Team Shanghai Alice booth, selling ZUN art-decorated
kouhaku manjuu: a pair of buns with sweet bean filling, one red-colored and the other white, which is a gift for ausipicious occasions. This is said ZUN art:

Translation:
紅白饅頭 Kouhaku manjuu
寿 Felicitations
饅頭でいいのね? Even selling manjuu is OK?
博麗神社例大祭 Hakurei Jinja Reitaisai