Multiple Json Files or A single file with multiple arrays

I would think this depends strongly on your use-case. The multiple files or multiple arrays you create will partition your data somehow: will the partitions be used mostly together or mostly separate? I.e. will there be a lot of cases in which you only read one or a small number of the partitions?

If the answer is "yes, I will usually only care about a small number of partitions" then creating multiple files will save you having to deal with most of your data on most of your calls. If the answer is "no, I will usually need either 1.) all/most of my data or 2.) data from all/most of my partitions" then you probably want to keep one file just to avoid having to open many files every time.

I'll add: in this latter case, it may well turn out that the file structure (one array vs an array-of-arrays) doesn't change things very much, since a full scan is a full scan is a full scan etc. If that's the case, then you may need to start thinking about how to move to the prior case where you partition your data so that your calls fall neatly within few partitions, or how to move to a different data format.