You can log into the minecraft launcher on one PC with as many accounts as you want. You can also install as many as you want on each computer you have. However your saved games are not syced between PCs. On the same PC though, each launcher will still be able to access the same saved worlds.
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You have 10 hearts with 2 health per heart, meaning a maximum health of 20
Its 1984 fps
Well the cheapest I have seen is from treepuncher they offer minecraft servers from $1 per player slot $1.50 per player slot if you want bukkit
It uses about 10 megabytes per hour if playing on downloaded version but when playing on the website version it takes about 20-30 megabytes.
Minecraft is a onetime Purchase not every game is a pay to play...pay to play meaning you pay per month to play.