Wikipedia states: "At the highest levels of the sport, a team of up to eleven officials may be on court at any given time".
The source of that information is given as this document from the International Tennis Federation:
http://www.itftennis.com/media/107082/107082.pdf
Section L, on page 22, thereof includes this: "If a Chair Umpire has a full complement of ten (10) Line Umpires, then the assignments/responsibilities shall be as shown above", which confirms that the
maximum number of umpires at a match under ITF rules can be 10.
However it should be noted that:
(a) the list of all 10 umpires includes those whose duties who are largely (if not wholly) taken over by electronic equipment at Wimbledon ; and
(b) the ITF rules allow for the use of only 5, 6 or 7 line umpires. (See Appendix D, on pages 34 to 36 for plans of their positions).
Further, ATP World Tour tournaments also have their own rules which state: "Unless otherwise approved by ATP, the following are required: A minimum of seven(7) line umpires must be provided per main draw match. A minimum of five (5) line umpires per qualifying match must be provided."
http://www.atpworldtour.com/~/media/files/rulebook/2015/2015_atp_rulebook_chapter_v.pdf
So the rules (of both the ITF and ATP) are met at Wimbledon if the chair umpire is assisted by 7 line umpires (making umpires 8 in all) but the same rules also permit the use of 10 line umpires (making 11 umpires in all).
Er, any clearer now?