Q1) What is the correct number depends on your workload, but having one file per CPU if you have 256 cores is unlikely to be a good idea. I've seen some formulas that I don't recall exactly. But when you install SQL Server, you can configure up front how many tempdb files you want. There is a default which is computed from what hardware you have. If still in doubt, go for eight files, but not more, unless you are actually seeing GAM contention.
Q2) Your attachment did not make it, so it is not clear what values you think should be less than 10-15 ms. But, yes, if the average time for a read or write operation exceeds 15 ms, this suggests that your I/O operation is not up to speed. This value of 15 ms comes from the access time you can expect on a spinning disk. These days we mainly have SSDs, and the access time in this case is often below the millisecond. But on the other end, we don't have a lot of local disks in our servers these days, but they are in a SAN, and the SAN and the network route to the SAN can be overtaxed, leading to access times way above 15 ms. Something from the perspective of a DBA is nothing acceptable.