How many addresses does my wallet.dat contain? Do I need to make a new backup every time I make a transaction? I'm having little problem here, trying to figure out the safest way to backup my wallet.dat to make sure that no BTC may be lost in case of recovery.

I'm not sure how the process of adding new addresses may affect a, say, a recently created backup.

So, basically, do I need to backup continuously to make sure that no BTC are lost? do I need to backup after each new address is created?

Can you provide me with some general golden rules when backing up wallet.dat?
By default, Bitcoin Core creates 100 addresses the first time it starts, and tries to keep 100 not-yet-used keys in wallet.dat 'keypool' (when you unlock the wallet to make a payment it will generate more).

So you do not need to backup continuously. The 'getinfo' RPC command will tell you the time when the oldest not-yet-used key was generated ('keypoololdest').

Any wallet backup created after the keypoololdest time will be good enough to recover all of the bitcoins sent to your wallet, but I'd suggest you figure out about how long it will take you to send and receive 100 transactions (e.g. two per day would be 50 days) and then backup about twice as often as that (maybe once a month if you're making 2 transactions per day).

Be sure your backups are encrypted with a strong passphrase, and make sure you store them someplace off-site that is safe.

And for long-term storage of more than one or two bitcoins, use a hardware or paper wallet.