You only need one for the "Roses vs Violets" achievement, but how many are there?
What are they, and what are their locations? Are they only in Spartan Ops?
SO FAR (Dec 04 2012) There are 5 known Easter Eggs:
You can find one in Spartan OPs Chapter 1, Mission 5: See how to easily obtain it here
You can find another in Spartan OPs Chapter 2, Mission 5: See how to easily obtain it here
You can find another in Spartan OPs Chapter 3, Mission 5: See how to easily obtain it here
You can find another in Spartan OPs Chapter 4, Mission 5: See how to easily obtain it here
You can find another in Spartan OPs Chapter 5, Mission 5: See how to easily obtain it here
A better answer and more information to come as people play and find more.
In mission 5 of chapter one clear out all the enemies to the point where it asks you to push the button to open the door. facing the button the ledge on your right overlooks lava and some rocks. on one of those rocks is a box that when shot plays the start of the blood gulch theme.
In episode 3 chapter 4 there is one, when you start pull out a scoped weapon and look at the top of the trees near where the ghosts are. There's a box up there, gets some funny dialogue when they bring in the Manti.
Episode 1 Chapter 5 and Episode 2 Chapter 5 so I'm assuming the 3rd one is Episode 3 Chapter 5.