No, for the most part Steam lets you download a game as many time as you want.
There are some cases, though, where a game will insist on using it's own DRM instead of Steam's, however (though the only one I can think of is Battlefield: Bad Company 2). I doubt Endless Space is going to be one of those games, so you should be fine in terms of that one at least.
As Strigion explains, Steam itself does not have any sort of "Only 5 installs" limit. Certain games may however have this (especially during the first few months/year of launch), though this is typicall only seen in EA games (Mass Effect, Spore) and such.
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