Splitting TempDB into multiples files equal to number of CPUs
A ratio of 1/4 to 1/2 times the number of TempDB data files to machine cores has long been the recommendation...


  But there's now even better guidance. At PASS in 2011, my good friend Bob Ward, who’s the top guy in SQL
  Product Support, espoused a new formula: if you have less than 8
  cores, use #files = #cores. If you have more than 8 cores, use 8 files
  and if you’re seeing in-memory contention, add 4 more files at a time. [link]


The last sentence has always been relevant. If you're not seeing contention, why add additional files? To play safe, most will add 2-4 files as a starting point for the majority of builds but beyond that, measure and react.