This one definitely depends on your situation. Most of the time I break the TempDB into only four or eight separate equally sized files with no auto grow. This has been a good general practice for my environment, but each server can be different depending on the type of data on them so testing is the best option. Also in the bulk of the tests I did a couple years ago I found no significant gains in splitting the TempDB files between different Luns/Spindles.