How many files should I create for one filegroup and 9 weekly partitions?
You need one file. 

Unless you have specific requirements to have multiple files, you only need one file for the filegroup. The 'specific' requirements would be things like requirement to spread the filegroup on multiple disk locations or a requirement for round-robin allocation. You did not mention any such requirement, you only give information about the number of rows in the tables, which is largely irrelevant when it comes to deciding the number of files in the filegroups.