I never saw it mentioned of 7 wizards.  There are 5 that I know of, Saruman, Gandalf, Radagast, and the Blue Wizards.  It HINTS that more may have come, and NOT landed in the northwest of Middle-earth, but never declares so.
   All the wizards were Maiar in Valinor, Gandalf's Maia name is Olorin, while Saruman's is Curumo.  To help the peoples of Middle-earth, the Valar sent them.  Saruman and one of the two Blue wizards volunteered (The other blue wizard followed his best friend), Radagast was asked to come by the Vala Yavanna, to look after animals and other living things; and Gandalf was asked to go by Manwe and Varda.  He (Olorin) was said to be the wisest of all the Maiar (Sauron was most likely the most powerful of the  Maiar).
   While in Middle-earth, Saruman and the two blue wizards wandered far into the east; Saruman came back but the others never did, and Tolkien never mentions what ever became of them. 
   As for the White Concil, Galadriel openly campaigned for Mithrandir (Gandalf) to get the top spot.  But he wouldn't do that, he wouldn't force himself to have any ties, and be forced to come if called.  He perfered to have no alleigances except to those in need.  So Saruman took up the position as the head, and was not at first evil.  It was his studying too deeply the dark arts of Sauron that led him to envy Sauron and wish to contend with him, and become as powerful. 
   But of all the Istari (wizards) sent to Middle-earth, ONLY Gandalf completed completed his task, and then was able to return to his home in Valinor. 
   But I think Tolkien only mentioned (for certain, not counting his vague hints of others MAYBE landing away southwards) 5 Istari.