The KS campaign starts at certain day and takes fixed time (like 30 or 40 days). And has set minimal amount goal.
During that time you have to "pledge" for desired reward (like "three accelerated Next models") - which will reserve the money from your card/account/whatever you use to pledge.
At the final day of campaign, if the project did reach the minimal goal, the money are transferred from KS to the project team, if the goal was not reached, the reserved money are returned back to the backers.
So there's finite period to reserve one, the number of machines is sort of infinite.
Because costs often scale with demand, campaigns often sport "stretch goals", promising further features/improvements to the original product depending on how much the original goal is shot over, like in case of first Next the larger FPGA chip and 1024kiB of SRAM for every machine (original proposal was one tier lower FPGA Spartan chip and 512kiB SRAM). So the more people throw money at the project, the more sweet extras may happen.
But at the base level you are reserving for fixed price some fixed reward, and the project either happens as whole, or you get your money back and team gets nothing, if the base goal is not reached.