12 Marines @4:30
From Liquipedia Starcraft 2 Wiki
An all-in timing attack at the 4:30 mark, plus rush distance, that utilizes the Supply Drop to get as many Marines as possible and then uses all SCVs available to tank damage, while MULEs continue to mine and support production for the three Barracks.
A good build to practice Bunker rushing and Marine/SCV micro during, but is easily countered by static defenses (Bunker, Cannon, Spine) or the opposing army not being small enough to kill with relative ease.
 Basic Build Order
- Clear Xel'Naga Tower with the first three Marines.
- Place the first Supply Depot and Barracks at the bottom of the ramp to wall off.
When moving out, keep SCVs in front of the Marines to tank. Use Marine micro in order to make the Marines cost effective as well as keep them alive. Although this build needs to win with the push, Calldown: MULEs could be used for some additional resources to produce extra Marines to tip it over. If the opponent defends with a relatively intact economy, you will be too far behind due to the loss of SCVs to catch up.
- Against Protoss, attempt to avoid the direct route in order to avoid being kited by Stalkers all the way back to the opponent's base.
- Against Zerg, if the build is scouted by an Overlord, push early and attempt to Bunker Rush. Also, rally your first Marines to the inside of your wall to defend against 6 Pool or 7 Pool.
- Against Terran, if scouted, the Bunker with SCVs repairing will be impossible to break, thus the game will be lost. Try to send your own SCVs up the ramp to surround the Bunker first to deny the opponent's SCVs the position to repair. If there is no Bunker, it should be relatively simple with proper micro.
 Pro features
- Any map with a 1 hex ramp that allows for blocking off at the bottom of the ramp with the Supply Depot and Barracks
- Short ground distance in order to hit the opponent before they have a sizable army or defenses.
 Con features
- Maps where the ramp cannot be walled off with a single Supply Depot and Barracks.
- MLG versions with a neutral Supply Depot.
- Geiko's TL Guide.
- 3-Rax all-in build demo by HalbyStarcraft, describes the strength and weaknesses of the push