I like @p.phidot's approach, but it relies on the file names to be consistent. Another approach is to use Power Query to combine all files in the same folder regardless of file name. It should work well, since you said you have the same columns in each file.
Here's an example with three files in a folder. Each has a different name but similar data, all in Column A, with varying occurrences of "RLL".
Here's what's in one of the files.
In a new file, go to the Data tab on the ribbon, and choose Get Data --> From File --> From Folder. Then select your folder.
On the Power Query popup, select Combine --> Combine and Load.
Select the columns you want to search and click "OK".
Then you can use a COUNTIF (or whatever suits you) to count the occurrences of "RLL".
This is a quick and easy way to get what you need and demonstrate the concept, but play with the other options too. Power Query is great for gathering data from lots of similar spreadsheets from a folder. Any time you add more spreadsheets, you just have to hit "Refresh" and it'll update.