I say that if a professional code breaker has a bunch of data, the encoded version of said data and the encoding method he can figure out the encoding key. It is just a matter of having enough data. Is this true?
How much data or how many characters (with matching coded characters) would a professional need in order to come up with the code key?
I assume you mean encryption when you say "encoding".
This model of attack is known as the known-plaintext attack. It is a situation where an attacker has samples of both the encrypted ciphertext and the corresponding plaintext. All commonly used encryption schemes thought to be secure are resistant against this form of attack (resistant meaning there is no better method than a naive bruteforce attack on the key).
In fact, most modern symmetric ciphers are secure against a much stronger model of attack, the chosen-plaintext attack where an attacker has the ability to encrypt arbitrary amounts of plaintext data and obtain the corresponding ciphertext.
So the answer to this is no, you are very wrong, at least when referring to encryption schemes that you should be using in real world situations.
1) Yes. This is called a known-plaintext attack, and is probably the simplest form of cryptanalysis.
2) Not much. Codebreakers working to defeat the Enigma cypher could figure out what the day's settings were with as few as four letters; for modern block cyphers, a single encryption block (eg. 128 bits of data for AES-128) is enough.
What you haven't asked is how long it would take. Most modern cyphers are highly resistant against known-plaintext attacks, such that the only one that works is brute force (ie. trying all possible encryption keys until you find the right one). For a cypher such as AES-128, the time needed for a brute-force attack is longer than the remaining life of the universe, even if the entire universe were turned into a codebreaking computer.
