For example, I play Emrakul, the Aeons Torn. That gives me an extra turn after this one. Then I activate Ral Zarek's ultimate and only get one extra turn (one) heads. Will the Emrakul's extra turn meld with Ral Zarek's so I only get 1 extra turn, or will I get two?
You get two extra turns.
500.7. Some effects can give a player extra turns. They do this by adding the turns directly after the current turn. If a player gets multiple extra turns or if multiple players get extra turns during a single turn, the extra turns are added one at a time. The most recently created turn will be taken first.