The private key will never change and does not depend on the number of transactions it is involved in. Saving the keystore file and the password is what you need to do for backup.
The name of the file has nothing to do with the private key; writing the name of the file doesn't backup anything. Again, see How do I backup my ether accounts?
eth ♦
- 86.5k
- 53
- 287
- 411