Merging multiple JSON files using Python
First off, if you want reusability, turn this into a function. The function should have it's respective arguments.
Secondly, instead of allocating a variable to store all of the JSON data to write, I'd recommend directly writing the contents of each of the files directly to the merged file. This will help prevent issues with memory.
Finally, I just have a few nitpicky tips on your variable naming. Preferably, head should have a name more along the lines of merged_files, and you shouldn't be using f as an iterator variable. Something like json_file would be better.