Starcraft – Will I keep the league placement if I don’t play any more than the five placement games

It sounds like you're building a large army and using it to gain map control, but then are falling prey to things like drops and backdooring.

If you have map control your first thought should never be: Crush my opponent, as you're essentially sacrificing an advantage (map control) for an unsure victory (because he doesn't control the map doesn't mean he can't take you). Instead your thought should be: take the map. I think you'll find this change in philosophy will lead to situations where you have a much more powerful economy than him and you're able to kill him with attrition.

Now if you're falling prey to things like BD here is a quick check list for you:

• Are you scouting? an Observer should be the first thing you build out of Robo. If you don't go Robo, a Hallucination works just as well. Sometimes scouting means sacrificing some Zealots in a pointless charge to just get inside his base.
• If you're scouting, are you seeing it coming? Overlords on the edge of your base, Starport with Reactor + 3 Rax, all of these are indicative of certain strategies you should watch out for.
• If you see it coming, how are you responding? If you have map control that means you should be able to respond quickly. Shutting down these back door attempts is often a bigger victory than a straight up conflict
• Finally, Do you have map control? If you don't have map control that means he does, and if he's beating you with map control then your problem is not "how do I deal with a base trade?" but rather, how do I regain the advantage.

Zerg expansions are the cheapest to build (costing only 350 minerals compared to 400) in addition they can grow their economy the fastest of any races. To compensate for their extreme economic power Zerg units are relatively fragile (largely due to their lack of range), not particularly cost effective, and the Zerg player must constantly choose between drones and army. In addition, one of the more technological (protoss or terran) races can support more overall production (workers + army) without being handicapped by the larvae mechanic. The combination of these force the Zerg to capitalize on its powerful economy to expand early (providing more larvae to match its opponents production) with the eventual goal of crushing the more technological races by sheer force.

Summary:

Zerg's 1 base cannot support as much production as 1 base terran or protoss due to having to choose between workers and army (larvae). Combined with cheap expansions and a strong economy it puts them into a balanced position in the match ups with a macro oriented play style