I have gone through 2 SSDs in my rMBP mid 2015 (16GB RAM). This was the last model with replaceable SSDs, I believe. I am on my third now:

  SMART/Health Information (NVMe Log 0x02)
  Critical Warning:                   0x00
  Temperature:                        38 Celsius
  Available Spare:                    100%
  Available Spare Threshold:          10%
  Percentage Used:                    7%
  Data Units Read:                    82,062,737 [42.0 TB]
  Data Units Written:                 81,415,281 [41.6 TB]
  Host Read Commands:                 539,567,311
  Host Write Commands:                561,934,808
  Controller Busy Time:               3,863
  Power Cycles:                       3,059
  Power On Hours:                     2,705