Thank you Peter!
Actually, I'm trying to create a access control, the numbers may be not going to be sequential, but randomically (by the way all the numbers will have 8 digits).
I was thinking about that Paul told me and you, and it's going to be hard to me to control random numbers with more files...
This project will have a TCP Client (Windows app) that will conect to the Arduino and send the 8 digits code that have the access on the system.
Maybe using a .txt file is a bad idea for that when you need to speed up the query data right ?
If any one have a better idea how to manage / handle that, I really appreciate that!