Hi,

I understand that planes [ideally] take off and land against the wind.

So for small airports w/ only 1 runway, do they build it to the direction that the wind is most likely to blow?

What about airports w/ 2 runways? Do they build them perpendicular to each other to maximize the chance of being able to take off and land against the wind?

Then why does SEA have 2 parallel runways? Land constaints?

Yes, the primary runway is oriented in line with the predominant wind direction at the site.

SEA has two parallel runways (and indeed a third in a few years) because of land constraints and because the wind rarely blows from the east or west quadrants (due to the mountains and ocean). Two runways both parallel to the prevailing winds (from the NNW during the summer, SSW during the winter) allows simultaneous landings and departures.

This isn't unusual at all. Note that LAX, SLC, DEN, AMS, ATL, DTW, and DFW all have 3-4 runways parallel to each other. Probably many others. They did it that way at DEN even though they had all the land they might want to build perpendicular runways. Way easier to operate an airfield when runways don't cross.

Airports with less traffic and multiple runways in places where the wind direction is distributed more evenly around the compass would tend to have two perpendicular runways (or three runways ~60 degrees from each other).

Actually in its original configuration SEA had three runways in a triangle.

As did LHR, which reflected wartime RAF practice. Then a second triangle was planned on top of the original triangle to give three sets of parallel runways. Over time, this has evolved into the current arrangement of a single pair of parallel runways oriented with the prevailing winds. The fact that they're parallel maximises the simultaneous use that can be made of them. LHR's crosswind runway (part of the original triangular configuration) still exists in theory but is "temporarily" out of use, has been for years, and is unlikely ever to be used again.