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, 18.104.22.168):
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.