The one database file per core recommendation is an outdated one for TempDB only. It's never been a recommendation for user databases and even for TempDB now it's 1/4-1/2 the number of cores, max 8 to start.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability