How is the TLS 1.2 sequence number generated? In the other ciphers supported by TLS 1.2, namely stream cipher and CBC block cipher mode, the sequence number is implicitly in the MAC (RFC5246, 6.2.3.1):

MAC = HMAC(MAC_write_key, seq_num +
                      TLSCompressed.type +
                      TLSCompressed.version +
                      TLSCompressed.length +
                      TLSCompressed.fragment);


How many bytes is seq_num? How is the sequence number generated? There seems to be no explicit information about this in the RFC document.
How many bytes is seq_num?


To quote the RFC (section 6.1; item 'sequence number')

 Sequence numbers are of type uint64


That is, they are 8 bytes long


  How is the sequence number generated?


From the same section:

 A sequence number is incremented after each
 record: specifically, the first record transmitted under a
 particular connection state MUST use sequence number 0.


That is, the sequence number for the first record is 0, the sequence number for the second record is 1, the sequence number for the hundredth record is 99.