How many packets does the IKE exchange use for Phase 1 Main Mode?

12 messages does not correspond to any standard IKE mode and is not defined by RFC 2409.

1 message is not a valid exchange count for any IKE mode since a minimum request-response pair is required.

3 messages describes IKE Phase 1 Aggressive Mode, which compresses the exchange at the cost of identity protection, not Main Mode.

IKE Phase 1 Main Mode exchanges 6 messages: messages 1 and 2 negotiate IKE SA parameters such as encryption algorithm, hash, DH group, and lifetime; messages 3 and 4 perform the Diffie-Hellman public value and nonce exchange to derive keying material; and messages 5 and 6 exchange identity and authentication data, which are encrypted using the keys derived in the prior exchange. This 6-message structure is what provides identity protection - a key advantage of Main Mode over Aggressive Mode.