The question of how many kanji are used in Japanese may have been answered roughly. The recently published supplement to the second edition of 日本国語大辞典 has a kanji and jukugo index to all of its headwords. The number of kanji that appear in word-initial position is 7068, plus the nonkanji 〆 and 々. (It is unclear how many other kanji appear only noninitially, and many more kanji presumably appear in proper names that aren't listed in the dictionary.)
The reading given for 々, by the way, is のま, from the character's resemblance to the katakana ノ and マ. The entry for のま has some interesting comments about 々 and similar characters, but there are no citations for the word のま itself.
(February 11, 2003)