1 head is one piece/branch of coral with a polyp in it. Each polyp contains a number of tentacles.
Try to buy one that has more than one head unless they guarantee the coral fragment you are buying is fine and has been on its own/separated for more than a month. Quite often shops will get in a big coral and break it up into fragments (frags) and sell the individual frags. They make more money from the frags and get to sell the coral to more people. This practice is most common in rare or unusually coloured corals.
The colour in the picture looks like the coral was photographed under actinic/blue lights. It probably won't look like that when you get it.