If you're using pre-shared key, then only 1 passphrase can be used at a time (the same one has to be entered on all devices connecting to the LAN). Since a passphrase can be anything, I would assume there would be an infinite number of possible passphrases. To quote the wikipedia entry on passphrases "This key may be entered either as a string of 64 hexadecimal digits, or as a passphrase of 8 to 63 printable ASCII characters. If ASCII characters are used, the 256 bit key is calculated by applying the PBKDF2 key derivation function to the passphrase, using the SSID as the salt and 4096 iterations of HMAC-SHA1." I'd suggest looking at the wikipedia entry for WPA for more info.