Do I need to generate transaction again after fork? How many confirmations are minimal to be sure of the transaction?
The transaction will need to be mined again as is likely that it will be still in the txpool of other nodes. In my experience, whenever a transaction is in an orphaned block I had to submit it again.
As for the number of transactions, 12 transactions is considered safe. See the accepted answer for this question for more details on the number of confirmations.
Hope this helps.