I wondered if we can use arduino uno to write to sd card only and only if the code on the rfid matches with the file in the sd card. And if it does not exist then it must do nothing. And how many maximum number of rfid cards can be using qith a single arduino Thankyou in advance
Yes, you should be able to do that.
I would start with an Arduino Uno and this RFID shield from Adafruit.
There is no limit on the number of RFID cards, only a limit to how much code / memory the Arduino has.