The A350 XWB is offering a great seating flexibility although the baseline of the A350-900 is 325 seats.
There are currently 4 operators and each one has different cabin configuration. And all of them with different seats comparing this typical capacity of 325.
Finnair is the operator with more business and premium economy seats (46+43), Qatar Airways is the company with less seats interior (283) while TAM is the airline with more seats in total (348).
Qatar (lowest passenger capacity) and TAM (highest one) have configured a 2-class interior.
Meanwhile Vietnam Airlines and Finnair have 3-class cabin.
Qatar Airways, the launch customer, has 36 business seats + 247 economy seats = 283 seats in total.
Vietnam Airlines has 29 business seats + 45 premium economy + 231 economy seats = 305 seats.
Finnair has 46 business seats + 43 economy comfort + 208 economy = 297 seats
TAM has 30 premium business + 318 economy seats = 348 seats. (almost in the first 3 aircraft)