What are good plans for getting the most units on the field within 5 minutes? There is only so much that you can do in 5 minutes, and many higher tech units are out of the question. What's the best build strategy for getting the most units very quickly? I'm thinking we can't spend quite as much on multiple production buildings because we want to save the money to make units in time for the cut off. Include all three races.
So before I start listing strategies I'd like to talk a little bit about "Rushes." For reference a strategy which involves an All-In within 5 minutes usually is called a "Rush." In order to make as large an army as possible in a 5 minute window you're going to make some large economic (and tech) sacrifices. This leads to two major problems:
The distance between bases is sometimes called the "Rush Distance." The reason this distance can be such an issue is because it gives your opponent time to prepare. Since a rush isn't going to be a extremely large force, this time can sometimes be enough for your opponent to build up a similar force.
That said, I don't want to discourage you from putting early pressure on an opponent, as it can often lead to a sufficient advantage and allow you to set the pace of the match. To that end here are a couple early pressure strategies which should not leave you at too much of a disadvantage:
The classic 2 Gate rush is an example of an early rush strategy which doesn't make too many sacrifices to place early pressure on the enemy. A two gate strategy is designed around gateways on 12 and 14 (probes) before adding on a second Pylon. This means you can Chrono Boost out 5 Zealots very easily. Its easy to underestimate the strength of a Zealot because its a Tier 1 unit, but your average Zealot has 150 combined HP/Shield, 1 armor and can 3 shot a Zergling. It is a very strong early unit.
The real strength of a 2 Gate (as opposed to rushing a Gateway on 10 probes) is that it transitions nicely into an early expansion (as you should have map control with your Zealots) or a 4 Gate or 3 Gate/Robo strategy. This means you can make a significant dent in your opponent's early economy without hurting your own too much. This should give you a nice advantage in the mid game (or even if your early push fails). This is not an All-In strategy which, if it fails, will spell game over for you.
Strangely building as large a force as possible is not the most powerful force an early Terran player can muster. Instead, building strong early game units and backing them up with Bunker play is far more powerful and effective. Additionally, it has the benefit of being able to salvage the bunkers later if necessary. In this category there are a number of good builds based around Baracks+Techlab into Early Marauders (or even Marines) + Bunker.
Your biggest weakness with this early play is going to be losing out on SCV production. Start with 12 Barracks, 13 Gas, this will give you the basics for your early Techlab. Keep those 3 SCVs on gas if you want to go Reapers or Marauders. If you're looking for more Marine based play, skip the Gas and get a second Barracks. You should be getting OC on 15-16, don't use the MULE. Since you're putting together an early rush, you'll get more Minerals out of boosting a supply depot. While a MULE is more minerals in the long run, upgrading a supply depot saves you 100 Minerals right away and an additional 60 that constitute the SCVs mining time.
When you do move out (usually with your first Marauder or 3 Marines), bring at least 1 SCV. You'll use these to build/repair bunkers. NOTE: currently you can repair a bunker from inside of it with an SCV. Its unclear if this is intended or will be patched later. There are a couple ways to use offensive bunkers. You can either try to slip them in while distracting with a Reaper, you can place them at the natural (to give you early map control), or you can place them at the entrance to his main (which will allow you to advance). There are a couple things you need to worry about:
The nice things about this rush is that it transitions nicely into 3 Rax (or 5 Rax Reapers) and if it fails you can easily salvage your bunkers.
There are a couple of rush strategies for Zerg, but many of them suffer from Zerg's weakness to choke points. That said, I've won many a game with early speedling pressure. Standard speedling play usually means 13/14 pool, 13/14 gas, 16 Queen. This means that Zergling Speed will finish right at the same time your first round of Larva spawn. As a result you can put out a large number of very fast Zerglings before your opponent has a chance to establish a standard build. This suffers from the fact that a wall can easily break a Zergling advance. Try to slip in if you can, if you can't target pylons and supply depots as they have the fewest hit points.
Alternately you can skip Zergling speed and build a Roach Warren on 18. This should allow you to push out 5 Roaches quickly. Similar to a 2 Gate build, these early Roaches are very powerful. After building your Roaches, you can either get Zergling Speed or power more drones. If you're just looking for early pressure, the drones are a good decision, if you're looking for a stronger push, the Zergling speed should allow Zerglings to quickly reinforce your roaches. In either case you want to expand fairly quickly or you won't be able to keep up with unit production. Additionally, the expansion will allow you to transition into mid-game easily, and leave you in a strong position.