Consider transferring the whole directory instead of individual files:
scp -r username@remote:/remote_path /local_path/
If that would transfer too much and you really only want to transfer the files whose name ends with .json in the single directory, you may want to consider rsync (which has better abilities for filtering what gets transferred):
rsync -av --include='*.json' --exclude='*' username@remote:/remote_path/ /local_path/
This would only copy files whose names end in .json but ignore other names. The terminating / on the source is needed here.