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as a rule of thumb after a backup you are safe to do up to 100 transactions.
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As a rule of thumb, after a backup you are safe until the sum of transactions you
send and
new addresses you create exceeds 100. Since it is possible that you might backup a damaged wallet.dat, it is much safer if you backup more frequently and keep copies of a few of the most recent backups. I generally suggest backing up when the sum of transactions you
send and
new addresses you create exceeds 25 since the last backup, and keeping copies of the 3 most recent backups.