You can download it as many times as you want, but if your account is not payed it will not recognize it as a valid login when you try to sign in on the game.
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You need to make an account and then pay for Minecraft and download it.
The account is free and you can tecnicly download the game without even buying it, but, in order for minecraft to work you have to pay for it for $26.95. Then you can download the .jar file.
You have to pay for it off: minecraft.net and then it will tell you from there.
You have to make a account and Buy the game it cost $26.95.
First, go to Minecraft's home page. The link can be found under "Sources and related links" below this answer. Then make an account for free, and click the download button. Now you'll have to pay a certain amount (the price changes a bit between £10 and £20, depending on the versions) to download it. Once you have bought the game, you can download it to as many computers as you want.