How many cabin crews are needed for an A350-1000?
It depends where you are flying, i.e. which countries rules apply. In the US for example, the minimum cabin crew is determined by the seating capacity:


  For airplanes having a seating capacity of more than 100 passengers—two flight attendants plus one additional flight attendant for each unit (or part of a unit) of 50 passenger seats above a seating capacity of 100 passengers.


(CFR 121.391 (a) (4))

The A350-1000 can seat up to 480 passengers, which would then require 10 flight attendants. Typical seating is however only 369 according to Wikipedia, which would only require 8 flight attendants.

In Europe, EASA defines the minimum number of cabin crew in the Type Certificate Data Sheet (TCDS):


  For the A350-1000, the minimum required cabin crew number established during the aircraft
  certification process is 8, irrespective of the Maximum Operational Passenger Seating Capacity
  (MOPSC).
  
  [...]
  
  In accordance with the operational requirement ORO.CC.100-Number and composition of cabin
  crew, if the MOPSC for the specific aircraft exceeds 400, the minimum required cabin crew number
  becomes 9.


(EASA A350 TCDS)

So here 8 or 9 flight attendants are required, also depending on the seating capacity.