Ok bunkers are great defensive structures. Though building them will slow you down, since 100 minerals can be used on something more important. Now building one bunker is usually good enough, since the strength of the bunker is in the marines and SCV, not the bunkers. Remeber building bunkers will slow you down, on building other strucutres as supply depots, command centers, and factories.
Now placing the bunkers is another topic. You usually want to place bunkers behind another structure as a barracks or supply depots, since these two are the only structures available early in the game. So try to combine both bunkers and supply depots at a your closest choke point. Another place to place your bunkers is inside your mineral field or to the left side of your command center. This is only to make the bunker practically invincible early in the game, as long as you can repair it. This is probably the best place to put it, since your opponent can run past your choke point bunker early in the game.
Now another thing about bunkers is that they increase the range of the marines by at least 1. I'm saying at least because there's a slight error on how the bunker increases the range. It will give your marines that one extra range when attacking from left and right of the bunker.
Now attacking from up and down, your marines will get an increase of 2 boxes. Strange eh? This has to do with the pixels of the graphics, and I'm not going to go into any further details about that. Now attacking down, has a slightly longer range than attacking up., but not something noticable, unless you have two bunkers, and one is attacking while the other can't. So placing bunkers so that they can attack upwards or downwards is better than left or right.
You should always have a few SCVs next to bunkers that are extremely close to the enemy. I'm talking about the bunkers that you are using to keep your enemy boxed in. You should have 5-6 SCVs, since this will protect the bunker long enough to allow your marines and tanks to finish off any units.
